Sift the flour into a bowl and mix with the yeast, salt, lemon zest and sugar. Add the softened butter in pieces.
Now knead with enough milk to make a smooth dough that should come off the edge of the bowl. Cover with a damp kitchen towel. Fill a larger bowl with warm water and place the bowl with the yeast dough in it. Let rise for about 1 hour.
In the meantime, cover a tray with baking paper.
Then knead the dough again on a lightly floured work surface, roll out about 1 cm thick and cut out about 12 triangles of the same size. Place about 1 teaspoon of Nutella on each triangle and roll up.
Place the croissants on the tray with plenty of space between them, brush them with a little beaten egg and sprinkle some sugar on them. Let rest for 15 minutes.
Meanwhile, heat the oven to 200 ° C. Bake the croissants for about 10-15 minutes until they are golden.