Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides to easily lift the bars out later. Lightly grease the parchment paper.

In a medium bowl, melt the 0.5 cup of unsalted butter (for the brownie layer). Stir in 1 cup granulated sugar until well combined. Whisk in 2 large eggs and 1 tsp vanilla extract until smooth. In a separate small bowl, whisk together 0.5 cup all-purpose flour, 0.33 cup unsweetened cocoa powder, and 0.25 tsp salt. Add the dry ingredients to the wet ingredients and mix just until combined – don't overmix! Fold in 0.5 cup semi-sweet chocolate chips.

In a large bowl, cream together the 0.5 cup softened unsalted butter (for the cookie layer), 0.25 cup granulated sugar, and 0.5 cup packed light brown sugar using an electric mixer or by hand until light and fluffy. Beat in 1 large egg and 1 tsp vanilla extract until well combined. In a separate bowl, whisk together 1.5 cups all-purpose flour, 0.5 tsp baking soda, and 0.25 tsp salt. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Fold in 1 cup semi-sweet chocolate chips.

Spread the brownie batter evenly into the prepared 9x13 inch pan. Take spoonfuls of the cookie dough and dollop them over the brownie layer. Gently spread the cookie dough to create an even, albeit slightly bumpy, top layer. Bake for 30-35 minutes, or until the edges are set and a toothpick inserted into the center comes out with moist crumbs, not wet batter. The cookie layer should be lightly golden.

Let the brookie bars cool completely in the pan on a wire rack for at least 2 hours. This is crucial for them to set properly and prevent them from falling apart when cut. Once cool, use the parchment paper overhang to lift the entire slab out of the pan and cut into 16 squares.