Put the water in a small saucepan and add the butter, bring to a boil. Add flour, then stir, add the eggs one by one and keep stirring! It will be very hard work, though. Add most of the cheese. Put the dough on a baking sheet, shape it into a doughnut (a hole in the center), glaze with egg yolk, and sprinkle the remaining cheese on top. Put in a hot oven 220 degrees for 30 mins. Serve on a large dish with salad in the center and eat as a starter. Serves 6.