• Preheat your oven to 325°F and position oven rack in the middle. Line an 8-inch square aluminum pan with parchment paper and set aside.
Using an aluminum pan ensures the brownies cook evenly.

Make Brownie Layer

• In a medium saucepan over medium-low heat, melt the butter. Once melted, whisk in the granulated sugar until smooth and shiny, about 2–3 minutes. Remove from heat and let it cool slightly.
  ½ cup (113 grams; 1 stick) unsalted butter, 1 cup (200 grams) granulated sugar
• Whisk in the whole egg and extra yolk until smooth. Then stir in cocoa powder, sourdough discard, vanilla extract, baking powder, and salt until well combined. Fold in the chocolate chips until mostly melted, then gently fold in the flour until just combined.
  1 large egg, 1 egg yolk, ½ cup (50 grams) cocoa powder, ¼ cup (50 grams) sourdough starter discard, 1 teaspoon vanilla extract, ½ teaspoon Diamond Crystal Kosher Salt, ½ teaspoon baking powder, 1 cup (170 grams) semi-sweet chocolate chips, ⅓ cup (40 grams) all-purpose flour
• Spread the brownie batter evenly in the prepared pan and set aside while you make the cookie dough.

Make Cookie Layer

• In a medium bowl, whisk together flour, salt, and baking soda. Set aside.
  1 ¼ cup (150 grams) all-purpose flour, ½ teaspoon Diamond Crystal Kosher Salt, ½ teaspoon baking soda
• In a large bowl, use a hand mixer or stand mixer to cream the softened butter, brown sugar, and granulated sugar until light and fluffy. Beat in the egg until fully incorporated. Then mix in the sourdough discard and vanilla extract until smooth.
  ½ cup (113 grams; 1 stick) unsalted butter, ⅓ cup (75 grams) brown sugar, ⅓ cup (75 grams) granulated sugar, 1 large egg, 3 tablespoons (38 grams) sourdough starter discard, 2 teaspoons vanilla extract
• Gradually add the flour mixture to the wet ingredients, stirring until just combined. Fold in the chocolate chips.
  1 cup (170 grams) semi-sweet chocolate chips

Assemble and Bake

• Dollop the cookie dough evenly over the brownie batter. Use a butter knife, skewer, or chopstick to swirl the layers together.
• Bake for about 60 minutes, or until a toothpick inserted into the center comes out clean.
If the top starts to brown too quickly, loosely cover with foil.
• Let the brookies cool in the pan for 1 hour, then transfer to a wire rack to finish cooling. Once completely cooled, slice into 16 squares and enjoy!