Start by lining a 22cm spring bottom cake tin with non-stick paper. Place the toasted walnuts into a food processor and blitz to a coarse crumb. Next, place the egg whites and 100g sugar into a bowl and whisk for 10 minutes to a soft meringue. Remove from the bowl and set aside. In the same bowl add the remaining 100g sugar, 4 egg yolks and vanilla and whisk together for 8-10 minutes until thick and glossy. Carefully fold in the walnuts, followed by the meringue. Spoon into the prepared tin, even out and bake in the oven at 200`c for 30-35 minutes. Meanwhile, make the espresso cream by whisking together the double cream, icing sugar and espresso to soft peaks. When ready, dust the walnut cake with icing sugar and serve in wedges with a large dollop of espresso cream on top and dusted with cocoa.