Put the crushed potatoes in a large bowl. Add the flour and mix well. Add the yeast dissolved in the milk and mix in the middle to form a pre-dough. Let the dough rise a little. Then add the sugar, egg, butter and the salt to the bowl and work everything into a fluffy yeast dough and let it rise until the dough has doubled.
Preheat the oven to 160 ° C convection.
Roll out the dough about 1cm thick on a floured baking mat. Cut triangles out of the dough, put a spoonful of jam on top, spread and roll up into croissants. Place the croissants on baking sheets lined with baking paper. Brush with beaten egg yolk and milk and let rise again (approx. 30 minutes) in the oven and bake for approx. 15-20 minutes.
Mix the powdered sugar with enough water until a viscous mass is formed, brush the croissants with it.