Combine the yeast, 1 teaspoon of sugar and the warm sour cream. Sift the flour into a bowl and add salt. Crumble in the room-temperature butter, and add the cooked and mashed cold potatoes. Add the egg yolks and the sour cream-yeast mixture. Knead the ingredients into a dough and rest it in a large plastic bag in the fridge overnight. On a floured surface roll out the dough to a ½-inch (1.2 cm) thick rectangle. Fold the dough over in thirds, turn it 90 degrees and fold it again in thirds so that it has 9 even layers. Cover it with a kitchen towel and let it rest for 20 minutes. After resting, roll out the dough into a 2/3-inch (1.5 cm) thick rectangle and score the top.
With a 2½-inch (6 cm) wide cookie cuter cut out the dough and place the pogácsa in a lined baking pan. Let it rest covered for 20 minutes, and brush the top with the egg. Bake it in a preheated 375 °F (190 °C) oven for approximately 25-30 minutes. Don’t make the dough too thick. The pogácsa will rise during baking; they will tumble over in the pan if they are too tall. I recommend using a digital kitchen scale for this recipe.