My favourite chocolate cake

Chocolate cake absolutely has to be two things: dark and moist. To make sure it is sticky and dense I use buttermilk. This might seem a curious ingredient to include, but it is completely necessary to guarantee a moist and tender crumb. Essentially cultured sour milk (not the same thing as soured, spoiled milk that has gone off), its acidity reacts with the bicarbonate of soda to give this cake its rise. If you can’t find buttermilk, use milk mixed with a little lemon juice as described in the recipe: the juice will make the milk curdle, but don’t worry about this. Adding coffee to chocolate cakes really brings out the flavour in the cocoa, but if you don’t like coffee or are baking for children, you can use boiling water instead. MAKES ONE 20CM SPONGE LAYER butter, for greasing 125g plain flour 225g caster sugar 50g cocoa powder 1 tsp bicarbonate of soda ¼ tsp salt 125ml buttermilk (cultured), or 110ml milk plus ½ tbsp lemon juice 60g butter, melted 1 large egg 125ml brewed coffee (or water) 1 You will need a 20cm loose-bottomed, deep cake tin. Preheat the oven to 180C/160C fan/gas 4 then grease the tin and line it with baking parchment. Put the plain flour, caster sugar, cocoa powder, bicarbonate of soda and salt in a large bowl. Whisk to combine. 2 If you don’t have buttermilk, pour the milk into a large jug and add the lemon juice. Leave to stand for five minutes until thickened. 3 Whisk the buttermilk/acidified milk with the melted butter and egg, followed by the coffee or water. 4 Pour the wet mixture into the dry ingredients, whisking until a smooth batter has formed. Pour this carefully into the prepared tin (it will be very runny). If you are making more than one layer, the best way to divide the batter between the tins is to transfer it to a large jug. You can then use either the markings on the jug to measure the amount going into each tin, or put a tin on the scales and measure by weight. It is important to do this so that the layers are the same height and cook at the same rate. 5 Bake for 25-30 minutes until risen and a skewer inserted comes out clean. Leave to cool in the tin for ten minutes before turning out on to a cooling rack. Bake two layers for this double- decker showpiece