Cupcake Preparation

1. Preheat the oven to 350°F (180°C) and line a muffin tin with 12 cupcake liners.
2. In a large mixing bowl, beat the softened butter with the sugar until light and creamy. Add the egg whites one at a time along with the vanilla extract, mixing well after each addition.
3. In another bowl, whisk together the flour, baking powder, and salt. Add this mixture alternately with the buttermilk: start with a bit of flour, then some buttermilk, and repeat until the cupcake batter is smooth and lump-free.
4. Divide the batter evenly into 6 bowls and add gel food coloring to each one (red, orange, yellow, green, blue, violet). Mix until the colors are bright and even. For easier layering, you can transfer each colored batter into small piping bags and pipe them neatly into the cupcake liners.
5. Fill the liners by layering the colors in rainbow order (violet, blue, green, yellow, orange, red).
6. Bake for 5 minutes at 350°F (180°C), then reduce the temperature to 320°F (160°C) and continue baking for another 15–18 minutes, until a toothpick inserted in the center comes out clean.
7. Allow the cupcakes to cool completely on a wire rack before frosting and decorating.

Mascarpone Whipped Cream

1. Whip together the mascarpone, cold heavy cream, powdered sugar, and vanilla until the frosting is thick and stable.

Assembly and Decoration

1. Pipe the mascarpone whipped cream on top of the cooled cupcakes.
2. Decorate with a rainbow candy belt and a few colorful sprinkles.