Boil the potatoes the day before and chill them overnight
Peel the potatoes, weigh 250 g and set aside. Cream fat, 125 g sugar and eggs with the whisk of the hand mixer. Mix flour and baking powder. Press potatoes through a potato press to the flour. Then add the butter-sugar mixture and work into a firm dough with your hands. Put the dough in a cool place for about 10 minutes
Mix the hazelnuts with 3 tbsp. sugar, bitter almond oil and milk to a paste
Roll out the dough into a circle (approx. 40 cm Ø) on a well floured work surface. Spread evenly with the hazelnut paste, leaving a rim of approx. 1 cm wide. Cut the dough circle into 12 cake pieces with a knife or dough wheel. Form each cake piece into a croissant by rolling it up from the wide side to the top
Place the croissants on a baking tray lined with baking paper and bake in a preheated oven (electric range: 175 °C/ convection oven: 150 °C/ gas: level 2) for 25-30 minutes. Remove from the oven and allow to cool for about 5 minutes. Put the croissants on a cake rack and let them cool down. Dust with icing sugar
waiting time approx. 12 1/2 hours