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Sourdough Cosmic Brownies

These Sourdough Cosmic Brownies are thick, fudgy, topped with glossy chocolate frosting and chocolate rainbow pieces. They taste like the lunchbox classic but richer, homemade, and so much better.
5 from 1 vote
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Prep Time 30 minutes
Cook Time 35 minutes
Servings 16

Equipment

  • 9×9-inch baking pan - to bake your brownies. I love this pan 9x9 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.

Ingredients
  

Brownie Base:

  • 1/2 (113g) cup 113g unsalted butter melted
  • 1/2 (120g) cup semi-sweet chocolate chips
  • 1/2 (42g) cup unsweetened cocoa powder
  • 1 1/2 (300g) cups granulated white sugar
  • 1/4 (50g) cup packed brown sugar
  • 1/2 (120g) cup sourdough discard 100% hydration
  • 3 large eggs
  • 2 tsp cornstarch
  • 1 tsp vanilla extract
  • 1 (125g) cup all-purpose flour
  • 1 1/2 tsp fine sea salt

Chocolate Frosting:

  • 3/4 (180g) cup semi-sweet chocolate chips
  • 3 (42g) Tbsp unsalted butter
  • 6 (90g) Tbsp heavy cream
  • 3/4 tsp vanilla extract
  • 1/8 tsp fine sea salt
  • rainbow chips for topping

Sourdough Discard Swap:

  • 1/4 (60g) cup whole milk
  • 1/4 (30g) cup all-purpose flour

Instructions
 

Step 1: Mix and Bake the Brownie

  • Preheat your oven to 350°F (175°C). Line a 9x9 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.
    1/2 (113g) cup 113g unsalted butter
  • 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.
    1/2 (120g) cup semi-sweet chocolate chips, 1/2 (42g) cup unsweetened cocoa powder
  • 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.
    1 1/2 (300g) cups granulated white sugar, 1/4 (50g) cup packed brown sugar, 1/2 (120g) cup sourdough discard, 3 large eggs, 1 tsp vanilla extract
  • 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.
    2 tsp cornstarch, 1 (125g) cup all-purpose flour, 1 1/2 tsp fine sea salt
  • 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.
    1/4 (60g) cup whole milk, 1/4 (30g) cup all-purpose flour
  • Spread the brownie batter into the prepared 9x9 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.
  • 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.
    6 (90g) Tbsp heavy cream
  • In another heat safe bowl, add the chocolate chips, butter, vanilla, and salt.
    3/4 (180g) cup semi-sweet chocolate chips, 3 (42g) Tbsp unsalted butter, 3/4 tsp vanilla extract, 1/8 tsp fine sea 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.
    rainbow chips
  • 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

Notes

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!

Nutrition

Serving: 1brownieCalories: 285kcalCarbohydrates: 40gProtein: 3gFat: 14g
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