In a large bowl, combine flour and yeast. Add butter, and using your hands mix and rub the butter in until the mixture resembles coarse sand. Add sugar, egg yolks and sour cream and blend well with your hands until the dough comes together in a ball. You’ll have to press it into shape. Do not overmix.
Wrap in plastic wrap and refrigerate 30 minutes or more. Remove dough and let soften a bit.
Preheat oven to 350 degrees.
Divide dough into four equal pieces and flatten each piece into a circle shape with your hands. Place between two pieces of parchment paper. On a large flat surface, roll dough out between the parchment paper sheets to create a circle, about 1/8-inch thick. If it’s perfect, good for you. If not, that’s fine.
Using a knife or pizza cutter, cut like a pizza into 8-12 pie-shaped wedges. On the wide end, add a scant teaspoon of apricot jam. Roll up toward the point and place on a baking sheet lined with parchment.
To make chocolate or cinnamon-nut, sprinkle those ingredients evenly over the rolled dough then proceed to roll up as above.
Beat an egg and brush over the tops. Bake until beginning to brown, about 20-30 minutes. Let cool a bit, and sprinkle, if you wish, with powdered sugar.