Grind the hazelnuts in a food processor or blender, or crush using a pestle and mortar, until fairly smooth but still retaining a little texture. Brown in a heavy-based frying pan over a low heat until evenly golden. Pour into a bowl and stir until cool.
Beat the butter and sugar together until the mixture is creamy. Add the flour and cooled nuts and mix well to form a soft dough.
Shape walnut-sized pieces of dough into balls, then pat into flat ovals. Place on a greased baking sheet and flatten slightly with a fork.
Bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 12 minutes, until just golden. Cool on a wire rack. They will keep, in an airtight container in a cool place, for a few days.