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 molds and bake at 350ºF for 30-35 minutes, or until risen and firm to the touch.
Place on a wire rack to cool for 10 minutes than turn out onto wire rack to cool completely.
Assemble as shown above using your favourite buttercream or ganache.
Notes
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