Preheat oven to 160 degrees C and line a baking tin with baking paper.
Beat together the flour, icing sugar and 80g butter, then beat in the egg yolk. Spread out the dough on the baking paper and press down well. Bake 25-30 minutes and let cool.
Meanwhile, in a saucepan combine the 2 tablespoons butter and chocolate. Add condensed milk. Cook over low heat, stirring, until chocolate has melted. Simmer gently about 5 minutes or until thickened. Stir in nuts.
Spread over the top of the shortbread base and chill 2 hours in the fridge before cutting into bars.