Preheat the oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper.

For the brownie layer, whisk together the melted butter, granulated sugar, brown sugar, eggs, and vanilla. Stir in the cocoa powder, flour, and salt until combined.

For the cookie layer, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy. Beat in the egg and vanilla. Add the flour, baking soda, and salt, then fold in the chocolate chips.

Spread the brownie batter into the prepared pan. Drop spoonfuls of cookie dough over the top and swirl gently.

Bake for 25-30 minutes, or until a toothpick comes out with a few moist crumbs. Cool before slicing.