Mix 1 1/5 cups (300 g) of flour, the yeast, salt and sugar well. In another bowl, mix the water with the powdered dry milk and stir until the milk dissolves.
Beat the mixture for another 4 minutes. Add the remaining flour gradually to it, in order to form an elastic dough. Remove the dough from the mixing bowl, cover it with a damp cloth and leave it in the refrigerator for between 2 and 12 hours, depending on how puffy you want the croissants to be. Divide the chilled dough into 3 balls.
On a floured surface, roll out each ball with a rolling pin. Cut out 8 triangles with a knife. Roll up the triangles into croissants and arrange them in a well-greased oven dish. Beat the second egg with a little water and spread it on the croissants.
Bake them for 18 minutes in a preheated 428°F (220 °C) oven. Serve the croissants with jam or butter.