Preheat the oven to 200C/180C Fan/Gas 6. Line an 8 inch square baking tin.
Combine the melted butter, granulated sugar and light brown sugar in a large mixing bowl or the bowl of a stand mixer. Combine on high speed for 3-5 minutes. Add the eggs and vanilla and combine again.
Fold in the flour and baking powder until all combined. Depending on how many colours you want, put a few tablespoons of the batter into small bowls/large mugs. Add a few drops food colouring to each and stir. You should end up with between 4-6 bowls of different coloured batter.
Add the batter to the lined pan one tablespoon at a time, taking from different colours for each scoop. Don't swirl together or do anything to combine. Once all the batter has been added, bake for 30 minutes.
Let cool on a wire rack before making the topping. Melt the white chocolate in the microwave in 20 second intervals, making sure it doesn't burn. Then pour over the cake. Add any sprinkles/glitter. Let set before slicing!