Bake the potatoes in their skins at about 200°C for 45-50 minutes, until easily pierced with a fork, then peel while still warm and press through a sieve or ricer.
Add the egg yolks, salt, the proofed yeast and the melted butter to the mashed potato.
Work in enough flour with a wooden spoon to get a firm, pastry-like dough. Knead well, then cover and leave to rise in a warm place for 40-50 minutes.
Meanwhile make the filling: mix the ground ham with the sour cream and pepper, salt to taste.
Turn the risen dough onto a floured surface, roll out to about 0.5 cm thick, and cut out circles with a larger round cutter.
Place a hazelnut-sized ball of ham filling in the centre of each circle, top with another cut-out circle, then cut out again with a smaller cutter so the filling is sealed tightly inside.
Cover the filled doughnuts and let rise for another 25-30 minutes.
Deep-fry in plenty of 170°C oil or lard until golden on both sides.
While still hot, roll immediately in grated parmesan and serve warm.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.