I found this recipe on the BBC food website and adapted it slightly, making cupcakes instead of muffins.
Ingredients (makes 30 cupcakes)
75g good quality cocoa powder (I used Green and Blacks)
180g plain flour
2 tsp baking powder
250g caster sugar
260g cooked beetroot
3 large eggs
170ml sunflower, corn or vegetable oil
1 tsp vanilla extract
3 tbsp icing sugar
1 tbsp (or more) beetroot juice
1. Preheat the oven to 180c and arrange the cupcake cases in cupcake tins or on a baking tray.
2. Sift the flour, cocoa powder and baking powder into a bowl and then add the sugar and set to one side.
3. Puree the beetroot (or mash with a fork) and then add the eggs one at a time. Then put in the vanilla and oil and mix well.
4. Make a well in the centre of the dry ingredients and gently add the beetroot mixture while stirring. Pour this into the cupcakes and bake for 15-20 minutes, until the top is firm when pressed with a finger.
5. Cool on a wire rack and then decorate with the icing sugar and beetroot juice mix, or just icing sugar.