Preheat the oven to 180C/350F/Gas 4. Grease a 20cm/8in cake tin with a little butter and line the bottom of the tin with a disc of baking parchment.
Blend the beetroot in a food processor to a rough purée.
Melt the chocolate in a bowl suspended over a pan of hot water (do not allow the bottom of the bowl to touch the water), then pour in the hot coffee.
Stir in the butter in small pieces and leave to soften. Remove from the heat and allow to cool slightly.
Meanwhile, sift the flour, baking powder and cocoa together in a bowl and set aside.
Separate the eggs. Whisk the yolks in a bowl until frothy. Stir the eggs into the chocolate and butter mixture, then fold in the beetroot.
Whisk the egg whites until still peaks form when the whisk is removed. Fold in the sugar.
Fold the sugar and egg whites into the chocolate mixture, then fold in the flour and cocoa mixture.
Pour the batter ointo the prepared tin and sprinkal cashew bake for 40 minutes, or until a skewer inserted into the cake comes out clean.
