Heat oven to 160C/140C/gas 3 and grease then line the base of two x 20cm sandwich tins with baking parchment. To start making the chocolate cream stir the yogurt with the cocoa and xylitol until completely blended then set aside while you make the cake. This helps to dissolve the xylitol granules.
To make the cake, first blitz the beetroot in a food processor until it resembles a thick puree. Tip in the cocoa, flours, ground almonds, xylitol, baking powder and soda and pulse briefly to mix the ingredients together.
Now add the eggs, the 150ml rapeseed oil, vanilla extract and milk, and blitz again to make a smooth liquid batter.
Divide the mixture evenly between the tins working quickly, as the baking powder activates once in contact with the liquid ingredients, then bake for 25-30 mins until a skewer poked into the middle of the cake comes out clean. Cool for few mins then remove from the tins and finish cooling on a wire rack. Once cold, carefully strip off the lining paper.
To finish the chocolate cream, whip the double cream until it holds its shape. Stir the cocoa mixture then fold in all but 2 tsp. Spread a third on top of one of the cold sponge cakes, top with the remaining sponge and spread with the rest of the chocolate cream to create a swirly finish. Dot over the reserved cocoa mixture and gently feather in with the end of a teaspoon. The cake will keep in the fridge for a couple of days, but return to room temperature before eating for the best taste and texture.