In a large bowl, cream together the butter, 4 tablespoons icing sugar and vanilla until smooth and light. Add the flour, salt and ground hazelnuts; stir together until well combined. Chill dough for 1 hour, then drop onto greased baking trays in rounded teaspoons.
Bake in preheated oven for 25 minutes. Allow to cool for 10 minutes, then roll in icing sugar. Cool completely on a wire rack, then roll in icing sugar once again.