For the dough mix flour, 125 g butter in flakes, 25 g sugar, salt, 1 egg yolk in a mixing bowl. Heat the milk in a small pot, remove from the heat, crumble the yeast into it and dissolve it while stirring. Knead with the flour mixture. Cover the dough and let it rise in a warm place for about 1 hour. In the meantime, bring 150 g sugar and 1/8 litre water to the boil for the filling, add hazelnuts and 50 g butter and mix well.
Let everything cool down. Knead the dough on a floured work surface with your hands again and roll out into a rectangle (45 x 30 cm) on the floured work surface. Cut into 6 strips of the same size. Cut the strips in half to make 12 rectangles of the same size. Cut each rectangle in half diagonally. Coat the triangles with the hazelnut filling, roll up from the narrow side to the top and bend into croissants. Place on 2 baking trays lined with baking paper. Whisk 1 egg yolk and cream. Brush the croissants with it.
Coat the triangles with the hazelnut filling, roll up from the narrow side to the top and bend into croissants. Place on 2 baking trays lined with baking paper. Whisk 1 egg yolk and cream. Brush the croissants with it. Bake the trays one after the other in a preheated oven (electric range: 175 °C/ convection oven: 150 °C/ gas: level 2) for about 20 minutes. Remove from the oven, allow to cool and dust with icing sugar
1 hour waiting time