Preheat your oven to 180ºC (fan oven) or gas mark 6. Grease and line 4 8" sandwich cake pans Cream together the butter and sugar until it’s a pale yellow colour and a fluffy texture. Slowly add in your eggs, one at a time. Add in the red food colouring and the yoghurt and beat everything until it’s mixed well. Sift the flour and cocoa together, then add half to the batter. Beat in the wet mixture and finally add the second half of the flour mixture. In a ramekin, mix together the bicarbonate of soda and the vinegar, which will fizz as you add them together. Add to your mixture immediately, folding everything in with a spatula. Distribute the mixture into 4 cake pans. If you don't have 4 pans, you can split the mixture between 2 pans and then slice in half once they have baked. If you are able to bake all at the same time, if not just bake your cakes two at a time. Place in the oven, on a medium shelf, and cook for 20-30 minutes, or until a skewer poked into the middle comes out clean. Once cooked turn out onto a wire rack to cool. Whilst the cake is cooling make the chocolate buttercream icing.