Preheat the oven to Gas 4, 180°C, fan160°C, then grease and line the bottom of a 23cm (8½ - 9in) springform cake tin. Sieve the cocoa, flour, baking powder and sugar into a large mixing bowl.
Drain and halve the beetroot, then blend in a food processor. With the machine running, add the eggs one at a time, then pour in the oil. Blend the mixture until the liquid is just smooth.
Stir the wet mixture into the dry ingredients and mix in the chocolate. Pour into the tin and cook for 45 minutes or until a skewer comes out clean.
Remove from the oven and set aside for 10 minutes, then turn out and leave to cool. Dust the cake with icing sugar before serving.
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