Step 1: Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
Step 2: For the brownie layer: In a medium saucepan, melt 1/2 cup unsalted butter over medium-low heat. Add 1 cup granulated sugar and 1/2 cup unsweetened cocoa powder, stirring until well combined and smooth. Remove from heat.
Step 3: Whisk in 1 teaspoon vanilla extract. Allow the mixture to cool slightly (about 5-10 minutes).
Step 4: Whisk in 2 large eggs one at a time, ensuring each is fully incorporated before adding the next. The mixture should be glossy.
Step 5: Gently fold in 1/2 cup all-purpose flour and 1/4 teaspoon salt until just combined. Do not overmix.
Step 6: Stir in 1/2 cup semi-sweet chocolate chips. Spread the brownie batter evenly into the prepared baking pan and set aside.
Step 7: For the cookie layer: In a large bowl, cream together 1/2 cup softened unsalted butter, 1/2 cup granulated sugar, and 1/4 cup light brown sugar with an electric mixer until light and fluffy.
Step 8: Beat in 1 large egg and 1 teaspoon vanilla extract until well combined.
Step 9: In a separate bowl, whisk together 1 1/4 cups all-purpose flour, 1/2 teaspoon baking soda, and 1/4 teaspoon salt. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.
Step 10: Fold in 1 cup semi-sweet chocolate chips.
Step 11: Drop spoonfuls of the cookie dough over the brownie batter in the pan. Gently spread and press the cookie dough to create an even layer over the brownie.
Step 12: Bake for 30-35 minutes, or until the edges are set and the cookie layer is lightly golden brown. A toothpick inserted into the brownie layer should come out with moist crumbs, not wet batter.
Step 13: Let the brookies cool completely in the pan on a wire rack before lifting out using the parchment paper overhang and slicing into squares. Enjoy!