Heat oven to 200C/180C fan/gas 6. Line 2 large baking sheets with baking parchment. Toast the hazelnuts in a dry frying pan, then grind in a food processor or mini chopper until roughly ground. Mix together the flour, sugar and ground nuts. Whisk the egg whites to soft peaks, then fold into the dry ingredients with the vanilla.
Drop tsps of the mixture, spaced apart, onto the lined baking sheets. Bake for 8-10 mins until just starting to colour. Let cool for 10 mins, then remove and cool completely on a wire rack. Store in a tin for up to 3 weeks.