Preheat oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper and lightly grease it.

Prepare the brownie layer: In a bowl, whisk melted butter and sugar until smooth. Add eggs and vanilla, mixing well. Stir in flour, cocoa powder, salt, and baking powder until combined. Spread evenly in the prepared pan.

Prepare the cookie layer: In another bowl, beat softened butter, brown sugar, and granulated sugar until creamy. Add egg and vanilla, mixing well. Stir in flour, baking soda, salt, and chocolate chips.

Drop spoonfuls of cookie dough over the brownie batter and gently spread without fully mixing.

Bake for 30-35 minutes until a toothpick inserted in the center comes out with a few moist crumbs.

Let cool completely before slicing into squares.