Sourdough Cosmic Brownies

If you loved those nostalgic snack cake brownies growing up, you need to make these Sourdough Cosmic Brownies. They’re thick, fudgy, topped with glossy chocolate frosting and rainbow pieces, and made with simple pantry staples. They taste like the lunchbox classic but richer, homemade, and so much better.

Why You’ll Love This Recipe

Thick, fudgy brownies: These bake up rich, chewy, and ultra chocolatey with that classic brownie texture.
Glossy chocolate frosting: This easy ganache frosting sets perfectly as the ultimate icing on the cake (or brownie in this case).
That signature rainbow topping: Rainbow candy-coated chocolate pieces make these brownies instantly recognizable and fun.
Uses up sourdough discard: Have discard to use? This is a delicious way to use it up!
Better than the store bought option: Homemade, simple ingredients, and extra fudgy. What isn’t there to love?

Tools You’ll Need:

9×9-inch baking pan- to bake your brownies. I love this pan 9×9 pan from USA Bakeware.
Parchment paper- to line your brownie pan for easy removal.
Mixing bowls- to mix the brownie batter and chocolate frosting.
Whisk- to mix the batter and frosting.
Spatula- to spread the brownie batter and chocolate frosting.
Digital kitchen scale (optional)– for accurate measuring, especially helpful with sourdough baking. I linked the one I like here.
Measuring cups and spoons– for measuring ingredients.
Oven safe metal clips (optional)- for holding the parchment down to the pan. I love these metal clips that are heat safe and keep my parchment from sliding around.

Gather Your Ingredients:

Sourdough discard– Adds flavor and moisture and is the magic that ferments your brownie batter. Can be used fed, past peak, or straight from the fridge.
Semi-sweet chocolate chips- used in the brownie batter and the chocolate frosting for extra chocolatey flavor.
Cocoa powder– smooth, dark flavor for that classic brownie intensity. I prefer to use a Dutch process cocoa powder, but any cocoa powder will work.
All-purpose flour– Provides structure to the brownies.
Unsalted Butter– Adds that classic chew to the brownies and adds silkiness to the ganache frosting.
White Granulated Sugar- Sweetens the brownie and gives that chewy texture and crackly top.
Brown Sugar- adds molasses flavor and chewiness.
Eggs– Helps the brownie bind and rise.
Cornstarch (optional)- adds chewiness and a slight cakiness to the brownies for that classic cosmic brownie texture. This is optional you can skip it if you’d like!
Vanilla extract– Adds warm cozy flavor to the brownie and frosting.
Sea Salt– Balances out the sweetness.
Heavy Cream- thins out the chocolate ganache frosting. You can substitute for milk but the frosting won’t be as thick.
Rainbow candy-coated chocolate chips- you can’t have cosmic brownies without the rainbow chips! I used these rainbow chips, but any sprinkles you’d like will work or M&M’s in a pinch.

Instructions

Step 1: Mix and Bake the Brownie

Preheat your oven to 350°F (175°C). Line a 9×9 metal pan with parchment paper, leaving some overhang so the brownies are easy to lift out after chilling. I like to use metal clips to hold my parchment in place. Lightly spray with cooking oil.

In a medium saucepan or microwave-safe bowl, melt the butter until just melted.

Add the semi-sweet chocolate chips and cocoa powder to the melted butter. Whisk over low heat, or microwave in short bursts, until the chocolate has just melted and the mixture is smooth.

Transfer from heat as soon as the chocolate has melted if you used a pot, then pour the mixture into a mixing bowl.

Whisk in the granulated sugar, brown sugar, sourdough discard, eggs, and vanilla extract until the brownie batter is shiny and smooth.

Add the flour, cornstarch, and salt to the brownie batter, and mix just until no streaks remain. Try not to overmix at this point or the brownies will be tough. You just want the flour incorporated.

Sourdough Discard Swap Option: If you don’t have sourdough discard, add the extra flour and milk at this time and mix it into the brownie batter.

Whisk the melted butter, cocoa, and chocolate chips

Spread the brownie batter into the prepared 9×9 pan, making sure it reaches the edges. Smooth the top with a spatula.

Overnight Fermentation Option: If you want to ferment this brownie batter, cover with plastic and refrigerate overnight in the pan. When you’re ready to bake your brownie, allow to rest at room temperature for about an hour to warm up then bake as instructed.

Transfer the brownies to the hot oven, and bake at 350°F for 35-40 minutes, or until the edges are set, the center no longer jiggles, and a toothpick inserted in the center comes out with moist fudgy crumbs.

Do not bake until the toothpick comes out completely clean. These brownies are meant to be fudgy and will continue to set as they cool. Baking time my vary on your oven, my brownies are done anywhere between 35-40 minutes.

Cool the brownies completely in the pan before frosting.

For the cleanest brownie texture, you can refrigerate the cooled brownies for 30–60 minutes before frosting.

Step 2: Frost and Chill the Brownies

While the brownie cools, prepare the chocolate frosting.

In a microwave safe bowl, heat the cream in the microwave or a small pot until steaming.

In another heat safe bowl, add the chocolate chips, butter, vanilla, and salt.

Pour the hot cream over the chocolate chip mixture, and cover for five minutes.

After the frosting has rested, stir with a whisk until it’s smooth, creamy, and glossy. If the chocolate hasn’t fully melted microwave for 15 second intervals and stir until the frosting is smooth.

Once the brownies are completely cooled, pour the frosting on top of the brownies and spread it evenly over the brownies to the edge.

Immediately top with the rainbow chip sprinkles while the frosting is still glossy and tacky.

Transfer the pan to the refrigerator, and chill for at least 2 hours, or until the frosting is set. I prefer to allow the brownie to chill overnight for it to set up cleanly and for the easiest cuts.

Condensation Note: If you chill the brownies overnight, make sure to cover them with plastic. Typically condensation can form on the top of the frosting if you don’t cover the pan. The condensation should stick to the plastic wrap overnight, and you can remove the plastic as soon as you remove the brownies from the refrigerator.

After the brownies have set up, remove from the refrigerator.

Use the parchment paper to lift the brownies out of the pan. Slice the brownies in half, and cut each half into 8 squares. I drew a line through the centers for that classic cosmic brownie look.

For the cleanest cuts, wipe the knife with a hot wet paper towel between slices.

Yields 8 cosmic brownies or 16 smaller brownies

How to Store:

Room Temperature- Store in an airtight container up to 3 days. Be careful about these brownies sitting in a warm environment, the frosting can melt.

Refrigerator- Keeps up to 5 days in an airtight container in the refrigerator. For the best texture, let them sit at room temperature for 10–15 minutes before serving.

Freezer- You can freeze the unfrosted brownies by wrapping them tightly and freezing for up to 2 months. Thaw in the refrigerator overnight, then frost as directed. Chill and enjoy!

FAQ’S

Why did my brownies turn out dry?

They were likely overbaked. For fudgy brownies, the toothpick should come out with moist crumbs, not completely clean.

Why are my brownies too gooey in the center?

They may need more cooling time or they were underbaked. These brownies are thick and fudgy, so they can seem underbaked if sliced warm.

Why is my frosting too soft?

For a firmer frosting, use heavy cream instead of milk and make sure the frosting has had time to set up in the refrigerator.

Can I use regular sprinkles instead of rainbow chips?

Yes, but rainbow chips give the most classic cosmic brownie look. Regular rainbow sprinkles will still work, but they may bleed slightly if added to warm frosting.

Can I make these without sourdough discard?

Yes, replace the 1/2 cup (120g) discard with 1/4 cup (60g) milk and 1/4 cup (30g) all-purpose flour.

How do I fix condensation on my frosting?

If you chill the brownies overnight, sometimes condensation can form. Best practice is to cover the pan with plastic and remove the plastic as soon as you remove the brownies from the refrigerator as condensation usually forms on the plastic. If you do have condensation, gently warm the top of the ganache with a very lightly preheated oven just until the frosting melts slightly and allow the frosting to chill 20 minutes before slicing. This usually works for me!

If you loved this recipe, drop a comment and leave a star review and don’t forget to checkout more of my sourdough chocolate recipes!

Sourdough Chocolate Recipes: