1 In a bowl, mix the banana with the powdered hazelnuts, sugar and milk. Add the butter cut into pieces. Place the bowl in a simmering water bath, stir until the butter is melted.
2 Roll out the puff pastry and cut out 8 rectangles of the same size. Place them in a cool place for 30 minutes. Grill the hazelnuts, crush them and put them in the fridge.
3 Preheat the oven to 210 °C (th 7). Place 4 rectangles of dough on the baking sheet covered with baking paper. Place banana spread on top and sprinkle with crushed hazelnuts.
4 Make horizontal cuts in the other rectangles of dough. Place them on the turnovers, welding the edges together. Dilute the egg yolk with a little water (or milk) and brush the turnovers with it. Bake for about 20 minutes