This cake is best eaten on the day it's made. If you make it a day ahead, ice it just before serving.
380 gmsoftened unsalted butter, plus extra for greasing380 gmcaster sugar380 gmblanched whole almonds, finely ground3vanilla beans, very finely chopped5large eggs200 gmwalnuts, finely chopped100 gm(2/3 cup) plain flour, plus extra for dusting½ tspbaking powder125 mlamarettoChocolate icing200 gmchocolate (at least 70% cocoa solids; The River Cafe uses Callebaut), coarsely chopped50 gmunsalted butter, coarsely chopped
Preheat oven to 160C, and butter and flour a 25cm-diameter cake tin. Beat butter and caster sugar in an electric mixer until pale and light (6-8 minutes). Stir in ground almonds and vanilla, then beat in eggs, one at a time, until combined. Fold in walnuts, flour and baking powder, then fold in amaretto. Spoon into cake tin and bake until a skewer withdraws clean (1¼-1½ hours). Set aside to cool briefly, then turn out onto a cake rack to cool completely.
Meanwhile, for chocolate icing, melt chocolate and butter in a bowl placed over a saucepan of simmering water, and stir occasionally until combined (3-4 minutes). Spread over cake with a hot, wet palette knife, cut into slices and serve.