Put the eggs, crème fraîche and sugar in a mixing bowl and whisk together until smooth. Add the ground hazelnuts, flour and baking powder and mix together.
Spoon the mixture into paper cake cases arranged in a 12-hole deep muffin tin and divide the blueberries evenly among them, pressing lightly into the mixture. Sprinkle with chopped hazelnuts.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for about 20 minutes until well risen and golden. Dust the tops with a little sifted icing sugar and leave to cool in the tin. These are best eaten on the day they are made.