Step 1: Preheat oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides.
Step 2: FOR THE BROWNIE LAYER: In a medium bowl, whisk together the melted butter and granulated sugar until well combined.
Step 3: Whisk in the eggs and vanilla extract until smooth and glossy.
Step 4: Sift in the cocoa powder, flour, baking powder, and salt. Gently fold with a spatula until just combined. Do not overmix.
Step 5: Stir in the semi-sweet chocolate chips. Pour the brownie batter into the prepared pan and spread evenly.
Step 6: FOR THE COOKIE LAYER: In a large bowl, cream together the softened butter, granulated sugar, and brown sugar with an electric mixer until light and fluffy.
Step 7: Beat in the egg and vanilla extract until well combined.
Step 8: In a separate bowl, whisk together the flour, baking soda, and salt.
Step 9: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.
Step 10: Stir in the semi-sweet chocolate chips.
Step 11: Drop spoonfuls of cookie dough over the brownie layer in the pan. Gently spread the cookie dough to cover the brownie layer as evenly as possible. It doesn't need to be perfect.
Step 12: Bake for 30-35 minutes, or until the edges of the cookie layer are golden brown and a toothpick inserted into the brownie layer comes out with moist crumbs (not wet batter).
Step 13: Let the brookies cool completely in the pan on a wire rack before lifting out by the parchment paper and cutting into squares. Cooling completely is crucial for clean cuts.