Heat the oven to 170C/fan 150C/fan 3. Line a deep 18cm loose-based or springform cake tin. Beat the butter and sugar together with electric beaters and then beat in the eggs, flour and baking powder.
Beat in 1 tbsp of the coffee mixture and then add up to another tbsp little by little until the mixture drops easily off the spoon. Keep the rest of the coffee mixture for the icing.
Stir in half the walnuts, snapping them in half as you drop them into the bowl. Spoon into the tin, level the top and bake for 40 minutes or until a skewer comes out cleanly. Cool.
To make the icing, beat the butter until soft and then beat in 200g icing sugar followed by the remaining coffee mixture, little by little. Stop when you have a depth of colour and flavour that you like. If the icing looks a little soft, beat in extra icing sugar.
5. Cut the cake into 3 slices horizontally and then sandwich the layers together with some of the icing, you need a reasonably thick layer. Ice the top of the cake with the rest of the icing and decorate with the rest of the walnuts.
I got this recipe from BBC Good Food. It's so easy to make (I made it while Poj was having his afternoon nap & iced it just before dinner - it's that easy).
I don't own a deep 18cm spring form cake tin, so I used my 2 ever so reliable 20cm loose-based cake tin instead. So I ended up with a 2-layer cake instead of 3 which worked just as well. But if you're going to make just 2 layers I'd suggest using only half of the icing mixture, as using too much could be really sickly.