First of all you need to toast all the walnuts, so spread them on a baking sheet and place in the pre-heated oven for 7-8 minutes.After that, reserve 10 halves to use as decoration later and finely chop the rest. Take a very large mixing bowl, put the flour and baking powder in a sieve and sift it into the bowl, holding the sieve high to give it a good airing as it goes down.Now all you do is simply add all the other cake ingredients (except the coffee and walnuts) to the bowl and, provided the butter is really soft, just go in with an electric hand whisk and whisk everything together until you have a smooth, well-combined mixture, then fold in the coffee and chopped walnuts. This will take about 1 minute but, if you don't have an electric hand whisk, you can use a wooden spoon and a little bit more effort. What you should end up with is a soft mixture that drops off the spoon easily when you give it a sharp tap; if not, add a spot of water. Divide the mixture between the prepared sandwich tins, spreading the mixture around evenly. Then place the tins on the centre shelf of the oven and bake them for 30 minutes.While the cakes are cooking you can make up the syrup and the filling and topping. For the syrup, first place the coffee and sugar in a heatproof jug, then measure 2 fl oz (55 ml) boiling water into it and stir briskly until the coffee and sugar have dissolved, which will take about 1 minute. Next, the filling and topping, and all you do here is place all the ingredients, except the reserved walnuts, in a bowl and whisk them together till thoroughly blended. Then cover the bowl with clingfilm and chill till needed.When the cakes are cooked, ie, feel springy in the centre, remove them from the oven but leave them in their tins and prick them all over with a skewer while they are still hot. Now spoon the syrup as evenly as possible over each one and leave them to soak up the liquid as they cool in their tins. When they are absolutely cold, turn them out very carefully and peel off the base papers – it's a good idea to turn one out on to the plate you're going to serve it on. Then spread half the filling and topping mixture over the first cake, place the other cake carefully on top and spread the other half over.Finally, arrange the reserved walnut halves in a circle all around. It's a good idea to chill the cake if you're not going to serve it immediately. You can also watch how to toast nuts in our Cookery School Video on this page.