Preheat oven to 350°F. Grease a 9x13-inch baking pan and line with parchment paper.

Start with the cookie layer: In a bowl, cream together 1/2 cup softened butter, granulated sugar, and 1/2 cup brown sugar until light and fluffy. Beat in the egg and vanilla.

Whisk together 1 1/4 cups flour, baking soda, and 1/4 teaspoon salt. Gradually mix into the creamed mixture. Fold in chocolate chips. Spread the dough evenly in the prepared pan.

For the brownie layer: In another bowl, combine 1/2 cup melted butter and 1 cup granulated sugar. Add 2 eggs and mix well. Stir in cocoa powder until combined.

Mix in 1/2 cup flour and 1/4 teaspoon salt until smooth. Pour the brownie batter over the cookie dough layer, spreading to cover.

Bake in the preheated oven for 35 minutes or until a toothpick inserted into the center comes out mostly clean. Cool in the pan on a wire rack.

Cut into squares and serve.