While the chocolate is melting, mix 85g self-raising flour, 85g plain flour, ¼ bicarbonate of soda, 200g light muscovado sugar, 200g golden caster sugar and 25g cocoa powder in a big bowl, mixing with your hands to get rid of any lumps. Beat 3 medium eggs in a bowl and stir in 75ml (5 tbsp) buttermilk.