1. In a bowl, combine 3 and 1/2 cups flour, 1/3 cup granulated sugar, and 2 tsp (or less, see note above) salt.
2. In a separate bowl, beat 3 egg yolks with 1 cup sour cream until combined.
3. Using a pastry blender, cut 1 stick of butter into the flour mixture until it resembles coarse breadcrumbs.
4. In a small bowl, mix 1 and 1/2 tbsp. active dry yeast, 1/4 cup lukewarm water, and 1/2 tsp. sugar. Allow to sit for 5 minutes until foamy.
5. Add the egg/sour cream mixture and the yeast mixture to the butter/flour mixture. Knead the dough until soft. (Do this on a lightly floured surface, but be careful not to add too much extra flour to the dough during kneading).
6. Refrigerate the kneaded dough for 1 and 1/2 hours.
7. After refrigerating, roll the dough into a 14 inch long oval. Spread on 2/3 of a can of Solo brand poppy seed pie filling, approximately 1/2 inch from the edges. Roll the dough beginning with the long end, and once completely rolled, pinch the ends together tight (this will help ensure that the filling will not leak during baking).
8. Preheat the oven to 350 degrees F.
9. Cover the poppy seed roll with a clean kitchen towel and let rise until doubled in size (30 minutes to 1 hour, depending on the temperature of your kitchen).
10. Brush with melted butter and bake for 30 minutes.
11. Once cooled, dust with powdered sugar.