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 two 8" pans and bake 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.
Once cooled, cut the circles out with the mould and assemble.