Pre-heat the oven to 170C/gas mark 3. You will also need two 20cm sponge cake tins, 4cm deep, lightly greased and the bases lined with silicone paper (20cm circles come in packs from Lakeland, John Lewis or any good kitchen shop).
First of all whiz the walnuts in a small processor (a mini-chopper will do this best) until they are very finely chopped. Now take a large mixing bowl and place the margarine, sugar and eggs in it, and sift the flour and baking powder in to join them – holding the sieve up high above the bowl to give the flour an airing. Next whisk the whole lot together with an electric hand whisk for about a minute, before adding the finely chopped walnuts. Give the mixture another quick whisk, then divide it between the two prepared tins and spread it out evenly, giving both tins a sharp tap.
Bake the cakes in the centre of the oven for 25-30 minutes or until the centres spring back when pressed with a little finger. Remove them from the oven, leaving them in their tins for a few moments before sliding a palette knife round the edges and turning them out on to a wire cooling tray, leaving the base papers on. To ice the cake, first choose seven of the best walnut halves for later, then roughly chop the rest.
Now carefully unroll the icing out flat, with its plastic cover intact underneath, and with a sharp knife trim away 3cm of icing all round. Combine these trimmings with the chopped walnuts to make a ball of walnut icing and roll this out to a 20cm circle.
Next warm the apricot glaze (or jam). Now carefully peel back the base papers on the sponges and place one sponge on a flat surface, then brush it with some of the glaze and cover this with the rolled-out circle of walnut icing. Brush the other sponge with some more glaze and place it on top of the first sponge, pressing it down firmly.
Finally brush the entire surface and side of the cake with the glaze (reserving just a small amount for later). Peel the round of icing away from the plastic and, with the help of a rolling pin, lift the icing on to the cake. Dust your hands with icing sugar and gently smooth the icing all over the top and sides of the cake, trimming away any excess from the edge.
Finally, using a dry pastry brush, brush away any excess icing sugar and use the rest of the glaze to fix the seven walnuts in a circle on top. This cake is best eaten fresh, but it will keep for a few days in an airtight tin or polythene box.