In a mixing bowl, cream the softened butter and sugar until smooth. Gradually add the eggs. Then add the flour and baking powder mixture, followed by the milk and vanilla.
Weigh the dough and divide it into 6 equal parts. Place them in 6 bowls.
Color each dough with one of the colors of the rainbow (purple, blue, green, yellow, orange, and red) using food coloring.
Bake each batter separately. Pour into a small 6-inch diameter cake pan and bake in 6 batches. Allow 10 to 12 minutes baking time for each batch in an oven preheated to 350°F.
Remove the dough circles from the molds and let them cool on a wire rack.
Using a large knife, cut off the top of each cake if it is domed, so that you have a smooth surface.
Combine the whole liquid cream and mascarpone in a mixing bowl. Mix with an electric whisk to whip the cream. When it begins to thicken, add the sugar and continue whisking until you have whipped cream.
To assemble the rainbow cake: place a circle of dough on the serving plate. Spread whipped cream on top and smooth with a spatula. Place another circle on top and spread with whipped cream again, smoothing with a spatula. Continue in this manner until all the circles have been used.
Finish the decoration by completely covering the cake with mascarpone whipped cream.
Add small dollops of whipped cream around the edge and base.
