Using extra butter and flour, grease a 25cm cake tin and line with parchment paper. (I used a 9" non-stick springform pan)
Dissolve the coffee in two tablespoons of boiling water. Chop the walnuts. Roast the hazelnuts in the preheated oven until brown (about 10 minutes), cool, then rub off the skins and grind to a fine powder. Finely chop the vanilla pods.
Beat together the butter and sugar until pale and light. Beat the eggs into the mixture one at a time. Stir in the hazelnuts, walnuts and vanilla pods. Fold in the flour and baking powder and finally stir in the coffee. Spoon into the prepared tin and bake in the preheated oven for an hour and a quarter. Allow to cool on a rack.