In a large mixing bowl, combine the softened unsalted butter and granulated white sugar. Using an electric mixer or stand mixer on medium speed, cream together for approximately 2 minutes until the mixture is light and fluffy. It'll be a bit crumbly, that's okay!
¾ (170g) cup unsalted butter, 1½ (300g) cups granulated white sugar
Add the egg, egg yolk, sourdough discard, and the vanilla extract to the bowl. Continue to mix on medium speed for another 2 minutes, ensuring all ingredients are well incorporated and fluffy. Remember to scrape down the sides of the bowl and mix everything at the bottom. This insures all your cookies come out evenly.
1 large egg, 1 large egg yolk, ½ (120g) cup sourdough discard, 2 (8g) tsp vanilla extract
Add the flour, baking powder, baking soda, salt, and cornstarch to the wet ingredients and mix very gently until the dry ingredients are just combined.
3¾ (470g) cups all-purpose flour, 1 tsp fine sea salt, 1 tsp baking soda, ½ tsp baking powder, ½ tsp cornstarch
Rinse and dry your fresh strawberries, and chop into small chunks.
½ (75g) cup fresh strawberries
Add the fresh strawberries and the chopped white chocolate to the cookie dough, then drop the strawberry jam in small spoonfuls all over the dough.
½ (120g) cup strawberry jam, 1 (175g) cup white chocolate pieces or chips
Gently fold all of the inclusions into the cookie dough or pulse them in using your mixer. Be careful not to overmix the inclusions at this stage, you want to leave pockets of jam through the dough.