Heat the oven 350°F/180°C. Grease a 8 inch (20cm) round pan and line the base with non stick parchment paper.
In a food processor grind the walnuts and the coffee until finely ground.
Beat the butter and sugar until pale and creamy. Beat in the eggs one at a time. Beat in the Amaretto. Mix in the walnut and coffee mixture. Sift together the flour, cocoa and baking powder. Add to the butter mixture and mix until just combined.
Spoon into prepare pan. Bake for 35-40 minutes. Remove from the oven and drizzle over 1 tablespoon Amaretto. Cool in pan. When cool remove from pan and place on serving plate. Mix remaining Amaretto with mascarpone and powdered sugar and spread over the top of the cake. Decorate as desired. Chill the cake for 30 minutes before serving.