PREHEAT OVEN TO 200ºC . Butter and flour an 20 cm cake pan. Spread hazelnuts on a cookie sheet and toast in oven for 5 minutes. Remove them and reduce oven to 180ºC . Grind nuts in a blender or food processor to a fine meal. Beat eggs and sugar together until pale yellow and creamy. Mix in flour, baking powder, salt and ground nuts. Transfer batter to prepared pan and bake 20-to-25 minutes, until edges of the cake begin to pull away from the pan. Cool on a rack and transfer to a plate. Just before serving, whip cream to soft peaks and spread it over top of cake.