• Start by getting the eggs to room temperature (you can do this quickly by covering them with warm water for 5 minutes). Melt the butter for the cookie dough (1/2 cup unsalted butter) and allow it to begin cooling.
• Preheat the oven to 350F (175C) and line an 8x8 inch baking dish with parchment paper, or use baking spray to grease it. Set aside.

Cookie Dough

• Combine the melted, cooled butter with the sugars in a medium mixing bowl and stir well. Add the sourdough starter, egg, and vanilla, and mix again.
• Sprinkle the flour, cornstarch, baking powder, and salt over the cookie dough batter and stir until just combined. Then fold in the chocolate chips. Set aside.

Brownie Batter

• Combine the butter, chocolate chips, and cocoa powder in a saucepan over medium-low heat, stirring frequently until smooth and completely melted. You can also use a microwave heating in 20-second intervals.
• To another medium mixing bowl, add the melted chocolate mixture and the sugars and mix well. Add in the sourdough starter, eggs, vanilla, and salt.
• Sprinkle the flour over the batter and stir until combined. Then fold in the mini chocolate chips if using.

Assembly

• In an alternating pattern dollop large spoonfuls of cookie dough and brownie batter into the baking dish. A cookie scoop works well for this. Use a butter knife to swirl both doughs together, keeping the batter in an evenly dispersed layer.
• Bake in a 350F (175C) oven for 30-35 minutes or until a toothpick inserted in the center comes out with a few moist crumbs (make sure you're not sticking the toothpick into a chocolate chip!). Overbaked brookies will be dry and crumbly, so check them at the 30-minute mark.
• Let the brookies cool completely in the pan before removing and slicing. Enjoy!