Peel the potatoes, chop them roughly and cook in salted water for 20 minutes (counting from the time they are cooked). Drain and let steam out. Squeeze through a potato press into a bowl. Knead into a dough with the flour and egg yolk. Season with marjoram, salt and white pepper.
Bring water to a boil in a large saucepan and season with salt. Using 2 tablespoons, shape the dough into roughly walnut-sized dumplings and pour them into the gently simmering water. When the gnocchi rise to the surface, skim them with a skimmer and place in a greased baking dish.
Pour the cream over the gnocchi in the baking dish and rub the cheese over it. Put the butter in small flakes on top. Bake at 220 ° C (top / bottom heat) for 15 minutes.