1 1/2 cup all-purpose flour
2 teaspoon baking powder
1 teaspoon salt
1 cup unsalted butter
6 ounces unsweetened chocolate
1 cup granulated sugar
1 cup light brown sugar
2 egg yolks
2 teaspoon vanilla extract
For the Cookie Dough:
3/4 cup unsalted butter
3/4 cup light brown sugar
3/4 cup granulated sugar
3 tablespoons whole milk
1 1/2 teaspoons vanilla extract
1 1/2 cups all-purpose flour
1 1/2 cups mini chocolate chips
Make the Brownies: Preheat oven to 350 degrees F. Butter an 9x13 baking dish and line with parchment paper, letting the excess hang over the edges. Butter the parchment paper; set aside.
In a small bowl, whisk together the flour, baking powder, and salt; set aside. Bring about an inch of water to a simmer in a medium saucepan.
Place a medium, heatproof bowl on top of the saucepan (do not let it touch the water) and add the butter and unsweetened chocolate. Stir occasionally, until the chocolate and butter are completely melted and smooth. Remove the bowl from the saucepan and whisk in the granulated and brown sugars.
Whisk in the egg, the yolk, and the vanilla extract. Add the flour mixture all at once, and fold it into the chocolate mixture with a rubber spatula until combined.
Pour the batter into the prepared pan and bake for 25 to 30 minutes, or until a thin knife inserted in the center comes out clean. Allow to cool completely; meanwhile, make the cookie dough.
Make the Cookie Dough: In a medium bowl, use an electric mixer on medium speed to combine the butter and both sugars. Add the milk and vanilla and mix until combined. Reduce the speed to low and mix in the flour just until combined. Using a rubber spatula, stir in the chocolate chips.
Spread the cookie dough over the cooled brownies. Refrigerate until the dough is firm, about an hour. Use a sharp knife or a pizza cutter (my favorite way to slice brownies!) to cut the brownies. Store the brownies in an airtight container at cool room temperature or in the refrigerator.