For the chocolate sponge
For the icing
- STEP 1
Heat oven to 180C/160C fan/gas 4. Oil and line the base and sides of a 33cm x 23cm roasting tin or cake tin with a lip of at least 2½ cm. Combine the flour, cocoa powder, baking powder, bicarb, sugar and a good pinch of salt in a large bowl. Rub any lumps of sugar between your fingers, shaking the bowl a few times to bring them to the surface.
- STEP 2
Whisk the oil, buttermilk, coffee, vanilla and eggs in a jug, then pour the wet ingredients into the dry. Use a spatula to stir well, removing any pockets of flour. Pour the mixture into the lined tin and bake for 25-30 mins until a skewer inserted into the centre comes out clean. If any wet cake mixture clings to the skewer, return the cake to the oven for 5 mins, then check again. Once cooked, remove from the oven and leave to cool in the tin for at least 20 mins.
- STEP 3
Meanwhile, make the icing. Melt the butter in a saucepan, then remove from the heat and stir in the icing sugar, cocoa powder and milk. The icing will be very runny but will thicken a little as it cools. (If the icing has set too much before the cake has cooled, reheat it slightly to make it easier to pour.)
- STEP 4
Pour the icing over the cake and leave to set. Decorate with the sweets, sprinkles, chocolate shavings or nuts, then cut into squares and dig in.