For the dough, dissolve the yeast, salt and sugar in the milk. Add egg and oil and stir well. Gradually add the flour and knead into a dough. Brush the dough with oil and let rise in a warm place for at least 1 hour until the volume has doubled.
Grate the cheese. Knead the dough again briefly and divide it into 5 equal parts. Roll out each piece to a thickness of a few millimeters and cut into 8 pieces like a cake (works well with the pizza cutter).
Take each triangle in your hand and pull it apart as thinly as possible. Cover the broad side with grated cheese and ham, bend the corners a little so that the filling cannot run out and roll up towards the tip so that you get a croissant.
Beat the egg yolks with the teaspoon of milk and brush the croissants with it. Bake in a preheated oven at 180 ° C for about 25 - 30 minutes until golden yellow. Tastes lukewarm best.