Preheat oven to 160 degrees C. Grease a 22cm springform cake tin and line with baking paper.
Place butter and chocolate in a bowl over a saucepan of boiling water, making sure the water does not touch the bowl. Melt the butter and chocolate, stirring constantly. Remove from heat and let cool 5 minutes. Stir in the zest, coffee and ground hazelnuts then stir well.
Meanwhile, using an electric beater, beat sugar and egg yolks until pale and creamy. Pour in the chocolate mixture and stir to combine well.
Whisk egg whites until stiff peak stage. Fold about 1/3 of the egg whites into the chocolate mixture. Add the remaining egg whites and fold quickly yet gently.
Spoon into the prepared tin and bake 40 minutes. Cool in the tin and then dust with cocoa powder.