Preheat the oven to 180°C.
Grease a 20cm cake tin with butter and line with parchment.
Purée the beetroot in a food processor until roughly smooth.
Melt the chocolate over a pan of hot water and stir in the hot coffee.
Stir in the butter pieces until softened and allow the mixture to cool slightly.
Sift the flour, baking powder, and cocoa powder together in a separate bowl.
Whisk the egg yolks until frothy and stir them into the chocolate mixture.
Fold the beetroot purée into the chocolate and egg mixture.
Whisk egg whites to stiff peaks and fold in the sugar.
Fold the egg white mixture and the sifted dry ingredients into the chocolate batter.
Pour the batter into the tin and bake for 40 minutes until a skewer comes out clean.
Cool the cake in the tin before serving with cream.