Rub together the butter, flour, egg yolks, salt, sugar and rum in a bowl.
Add enough sour cream to form a dough that is slightly firmer than strudel dough but still workable.
Knead by hand into a smooth dough, wrap in cling film, and chill in the fridge for at least 30 minutes until well chilled.
On a floured board, roll out and fold the dough four times in succession: roll out, fold into thirds, turn, roll out again - this creates the layered effect. On the last roll, roll to the thickness of a knife back (about 3-4 mm).
Cut the dough into equal strips, about 2x8 cm.
Fry in 170°C oil or lard until golden brown on both sides, about 2-3 minutes per batch.
Drain on paper towels, and while still warm, toss in cinnamon sugar or dust generously with it.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.