First, make sure all ingredients are at room temperature. Preheat the oven to 200 C / Gas 6 and grease and flour 6 23cm round cake tins.
Sift together the flour, baking powder and bicarb.
In a bowl, beat the butter, sugar, milk, egg whites and vanilla extract until smooth. Stir in dry ingredients, mix well.
Divide the mixture into six bowls. Mix in different food colouring to each bowl. Start with just a little bit of colouring, then mix; you can always add more!
Bake each for 15 to 20 minutes. Set aside to cool.
Icing
Beat the cream until stiff. Fold in icing sugar and mascarpone cheese, stir until smooth.
Spread icing on each layer of cooled cake, placing one on top of the other. Cover the top and sides with the remaining icing. Refrigerate for 2 hours before serving.