Preheat the oven to 350 F. In a large bowl, whisk melted butter with brown sugar, eggs, and vanilla extract until smooth. Add in the flour, salt, and baking powder until smooth.
Divide the mixture into three bowls, make one red and one blue. Add each color to a greased 8-inch square cake pan. Swirl with a knife. Bake for 20-25 minutes.