Preheat the oven to 180 C / Gas 4. Grease 2 (20cm) cake tins and line with baking parchment.
For the cake:
In a mixing bowl, beat the butter and sugar until light and fluffy. Beat in the eggs one at a time until pale and creamy. Add the coffee and mix until well integrated.
Sift the flour and baking powder and fold in, maintaining as much of the air as possible. Divide the mixture equally between the two prepared cake tins.
Bake for 20 to 25 minutes in the oven or until the skewer comes out clean. Remove from the oven and allow to cool completely.
For the buttercream icing:
Meanwhile combine the icing sugar, butter, vanilla extract and coffee in a bowl and mix with an electric mixer. Whisk until rather creamy. If too stiff add milk.
If you want to pipe, at this point, place the buttercream in a piping bag or you could just spread it all over the cake. Place the other cake on top and spread the buttercream on top. Decorate with coffee beans or just leave it as it is.
If you want it to be a coffee and walnut cake add 115g of walnuts and decorate with halved walnuts.