Quickly work the flour, the cold butter cubes, the egg yolk, a pinch of salt and the sour cream together until you get an elastic, strudel-like dough, then chill for 15 minutes.
Preheat the oven to 180°C.
Roll the dough out thin, and thinly line 4 small (about 8-10 cm) tart or muffin molds with it.
Fill the lined molds with parchment paper and dried beans or baking weights (blind baking), so the pastry doesn't puff up.
Blind-bake for 12-15 minutes, until the edges turn lightly golden, then remove the beans/paper, and let the baskets cool.
Turn the oven up to 220°C.
Sprinkle the bottom of the cooled baskets with the grated parmesan, then crack an egg into each, keeping the yolk whole, and salt the tops.
Bake in the hot, 220°C oven for 6-8 minutes, until the white sets but the yolk stays soft, and serve immediately, warm.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.