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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.
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.
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.)
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.