Start off by sifting the flour and baking powder into a roomy mixing bowl, holding the sieve quite high to give the flour a good airing as it goes down, then add the butter, eggs, caster sugar and coffee powder.Now, using an electric hand whisk, mix to a smooth, creamy consistency for about 1 minute. After that take a tablespoon and fold the chopped nuts into the mixture. Next divide the mixture between the two prepared tins, level off using the back of a tablespoon and bake near the centre of the oven for about 25 minutes. The sponges are cooked when you press lightly with your little finger and the centre springs back.Remove them from the oven and after about 30 seconds loosen the edges by sliding a palette knife all round then turn them out onto a wire cooling tray. Now carefully peel back the lining by gently pulling it back. Then lightly place the other cooling tray on top and just flip them both over so that the tops are facing upwards (this is to prevent them sticking to the cooling tray).While the cakes are cooling, make up the filling: in a small bowl combine the mascarpone, coffee powder and caster sugar with 1 tablespoon of milk – what you need is a smooth spreadable consistency. As some mascarpones are wetter than others it’s impossible to be precise, but add a bit more milk if you think it needs it. When the cakes are cold, spread half the filling over one, sandwich them together, then spread the rest over the top using a palette knife and making a swirling pattern. Then finish off by placing the walnuts in a circle near the edge. Store in a polythene box in the fridge.