Step 1

Preheat the oven to 180°C, gas mark 4. Lightly oil a rectangular baking tin, about 34x24x5cm, then line the base with baking parchment. For the cake, beat the oil, butter and sugar together with an electric mixer for 2-3 minutes until soft and paler in colour.

Step 2

Sift the flour and cocoa powder together in a bowl, then beat the coffee into the butter mixture, followed by the eggs one at a time. Beat well between each egg and add a spoonful of flour each time to stop the mixture curdling. Gently beat in the remaining flour on a low speed until just incorporated.

Step 3

Once the mixture is smooth, transfer to the tin, level the top and bake for 30 minutes, until risen and a skewer inserted into the centre of the cake comes out clean. Leave to cool in the tin for 10 minutes, then invert onto a wire rack, peel off the paper and leave to cool completely.

Step 4

For the buttercream, beat the butter in a bowl using an electric mixer for 5-6 minutes, until almost white, fluffy and soft. Sift the icing sugar into the butter in three additions, taking care to mix slowly after each addition before turning up the speed. Once all the sugar is in, whisk for 1 minute more. Reduce the speed and beat in the liqueur with a generous pinch of sea salt.

Step 5

Once the cake has cooled completely, spoon over the buttercream, then spread it with a palette knife. Scatter with sprinkles, if using, allow to stand for at least 30 minutes, then slice and serve.