Preheat oven to 375 F. Place the butter, water, and cocoa in a small saucepan and heat gently, without boiling, stirring until melted and smooth. Remove from heat and let cool slightly.
Beat together the eggs, buttermilk, vanilla extract, vinegar and beet juice until frothy. Beat in the butter mixture. Sift together the flour, cornstarch and baking powder, then stir into the mixture with the superfine sugar.
Pour batter into pan of your choosing and bake at 350 F for 30-35 minutes, or until risen and firm to the touch. Cool in pan for about 10 minutes, then place on a wire rack and cool completely before frosting.
For the frosting, I whipped 1 cup of heavy cream with 2 tablespoon icing sugar, coloured with a small drop of AmeriColor sky blue and decorated close to serving time.