1 bar (4-oz) bittersweet baking chocolate
1 stick butter
2 tbsp cocoa powder
1 1/4 cups sugar
1/4 tsp salt
3 large eggs
1 tsp vanilla extract
1 cup all-purpose flour
1/2 tsp baking powder
1 cup semisweet chocolate baking chips
1 cup white chocolate baking chips
Preheat oven to 350 degrees.
Line a 9-inch square pan with foil, leaving foil hanging over the sides. Spray foil with nonstick cooking spray; set aside.
Chop the bittersweet chocolate into small pieces. Place the chocolate, butter, and cocoa powder into a microwave-safe bowl. Microwave on high setting until both the butter and chocolate are melted. Stir to combine; set aside.
Place the sugar and salt into a medium mixing bowl. Stir in the chocolate mixture until combined. Add the eggs, one at a time, stirring after each addition until mixed in. Stir in the vanilla.
Stir the flour and baking powder into the batter; mix well.
Spread the batter in the prepared pan and bake at 350 degrees for 30 minutes (do not overbake). Remove pan from oven.
Sprinkle the topping chips over the hot brownies and return to oven for 1 minute. Remove from oven and allow to sit for 1 minute.
Using the foil handles, life brownies out of the pan. Using a knife or decorating spatula, swirl chips for a marbled effect.
Allow to cool completely before cutting into 20 pieces.