- 3/4 cup butter, softened
- 1 cup white sugar
- 1 cup firmly packed light brown sugar
- 3 large eggs
- 1 teaspoon vanilla extract
- 2 1/2 cups all-purpose flour, divided
- 2 1/2 teaspoons baking powder
- 1/2 teaspoon salt
- 1/3 cup unsweetened cocoa powder
- 1 tablespoon melted butter
- 1/2 cup milk chocolate chips
- 1/2 cup white chocolate chips
- Preheat oven to 350 degrees F (175 degrees C). Lightly grease a 9×13-inch baking pan.
- Beat 3/4 cup butter, white sugar, and brown sugar together in a bowl using an electric mixer until light and fluffy; beat in eggs and vanilla extract.
- Mix 2 1/4 cup flour, baking powder, and salt together in a separate bowl; stir flour mixture into creamed butter mixture until batter is smooth. Divide batter into two bowls.
- Whisk cocoa powder and 1 tablespoon melted butter together in a bowl; stir into 1 of the bowls of batter. This will be the 'black half'. Pour black half into the prepared baking pan.
- Stir the remaining 1/4 cup flour and milk chocolate chips into the remaining bowl of batter creating the 'white half'. Spread white half evenly over black half. Sprinkle white chocolate chips over white half.
- Bake in the preheated oven until edges begin to pull away from the sides of the pan, 20 to 30 minutes. Cool completely before cutting into bars.