With an electric mixer beat the butter and cream cheese until creamy. Add sugar and beat until fluffy, about 2 minutes. Beat in lemon zest and vanilla.
In a separate bowl, sift together flour and baking powder and stir into the cream cheese mixture. Add the nuts and chopped chocolate. Chill the dough in the fridge 2 hours.
Preheat oven to 200 degrees C. Lightly butter two baking trays. Drop heaping spoonfuls of the dough on the trays about 5cm apart and bake until golden, about 8 minutes. Let cool on wire racks then dust with icing sugar.