Preheat the oven to fan 160C/conventional 180C/gas 4. Melt the butter in a small bowl and leave to cool. Butter a shallow tin about 20 x 24cm. In another bowl, beat the eggs. Melt the butter, then add to eggs with the sugar and vanilla. Tip in the flour and stir in with the chocolate and three-quarters of the nuts. Don’t over-beat.
Spread the mixture in the tin, scatter over the rest of the nuts and bake for 25-30 minutes, then cut into 12 bars. Dust with icing sugar.