Heat oven to 180C/160C fan/gas 4. Make the first batch of sponges by greasing and lining two 20cm round tins. Gently melt 125g butter and 100g chocolate together in a saucepan. Mix 250g flour, 250g sugar, 1 tbsp cocoa, ½ tsp bicarb and ¼ tsp salt in a large mixing bowl. Whizz one egg with 100g yogurt and 200g beetroot in a food processor or blender until fairly smooth. Put the kettle on.
Tip the beetroot mix into the dry ingredients along with the melted chocolate mixture and 150ml boiling water, then stir to combine. Stir in half the food colouring, if using, and divide the mixture between the tins. Bake for 25 mins until a skewer poked into the centre comes out clean. Leave the cakes on a wire rack to cool completely.
Repeat Steps 1 and 2 to make two more sponges – or if you're lucky enough to have four 20cm sandwich tins, you can bake in a big batch all at once.
To make the frosting, briefly beat together the cream cheese and butter, then beat in the icing sugar and vanilla. Use a little to sandwich the cooled cakes together, then swirl the rest all over the sandwiched cakes and decorate with sweets, if you like. Sit the cake somewhere cool (not the fridge) to set a little before serving.