Sauté the leek in a frying pan with a little butter, transfer to a sieve and drain. Beat soft butter and yolks until creamy and stir in the cornstarch. Add well squeezed ricotta (it should have a crumbly consistency), bread rolls, grated lemon zest and sauteed leek to the creamed mixture form and mix everything together well. Rest for ten min to allow the bread to swell. Season the mixture with salt and freshly ground pepper and form into small dumplings. Cook in boiling salted water for about 15 minutes.
For the sauce, sauté shallot with butter in a saucepan, dust with flour and pour in the milk, whisk until smooth. Boil well for 2-3 minutes. Remove the saucepan from the stove, stir in the cheese cubes and let them melt. Season to taste with salt, pepper and a pinch of nutmeg.