Start by greasing and lining 2 x 20cm round cake tins. To make the coffee cake, dissolve the coffee granules in the hot water and put aside to cool. Make the cake as a Victoria sponge by beating together the butter and sugar till light, then add the eggs one at a time, then finally fold in the flour, baking powder, chopped walnuts, orange zest and dissolved coffee granules. Put the cake mixture into the prepared cake tins, level out, and bake in the oven at 200`c, for about 35 minutes, or until the cake springs back when pressed. Remove the cakes from the oven and leave to cool on a wire rack. Next, while the cakes cool down, you can prepare the coffee cream filling. Put the remaining coffee granules and coffee liqueur into a saucepan and heat gently to dissolve the coffee granules. Leave to cool completely. Next, place the butter in a bowl and beat till light and fluffy. Slowly stir in the icing sugar, and then add in the dissolved coffee and liqueur and continue mixing till you a have a smooth cream filling. Cut both of the cakes in half horizontally. Fill both cakes with 1/2 of the coffee cream filling, and then use 1/2 of the remaining cream to sandwich the 2 cakes together. Place the cake onto a large round serving plate and spread the remaining cream filling on the top. Finally, decorate the top of the coffee cake with walnut halves. For best results, chill the coffee and walnut cake before serving.