Preheat oven to 350 degrees F (175 degrees C).

Line a 13x9-inch pan with foil, extending the foil over the sides to create handles.

Spray the foil-lined pan with cooking spray.

In a large bowl, beat the softened butter and brown sugar with an electric mixer until light and fluffy.

Blend in the eggs, flour, and baking powder until well combined.

Chop 5 ounces of white chocolate into small pieces.

Stir the chopped white chocolate and chopped peanuts into the batter.

Spread the batter evenly onto the bottom of the prepared pan.

Bake for 20 to 25 minutes, or until a toothpick inserted into the center comes out clean.

Cool completely in the pan.

Microwave the remaining 1 ounce of white chocolate according to the package directions until melted and smooth.

Drizzle the melted white chocolate over the cooled dessert.

Melt the semi-sweet chocolate and drizzle it over the dessert.

Let the chocolate stand until firm.

Use the foil handles to lift the dessert from the pan.

Cut into triangles to serve.