Pirozhki

In a small bowl, stir together yeast, sugar, and 1⁄4 cup warm water; let stand until foamy, about 10 minutes. In a large bowl, using your hands or a fork, cut the 8 tbsp. of butter into the flour until it resembles coarse bread crumbs. Add the yeast mixture, sour cream, 1⁄2 tsp. salt, and the whole egg and stir until just well combined. Wrap dough loosely with plastic wrap and refrigerate for at least 2 hours or up to overnight.

Bring the dough to room temperature and grease a large bowl with the remaining butter. On a lightly floured work surface, knead the dough, adding small amounts of flour as needed to prevent sticking, until soft and smooth, about 5 minutes. Transfer to the greased bowl, cover with plastic wrap, and set in a warm place to let rest until doubled in size, about 1 1⁄2 hours.

Heat oven to 400°. In a large bowl, combine the hard-boiled eggs and scallions; season egg mixture with salt and pepper. Divide the dough into 3 equal portions. On a lightly floured surface, roll 1 portion of dough into a 1⁄8"-thick rectangle. With a 3" round cookie cutter, cut dough into 10 circles; reserve scraps. Gently stretch each circle of dough with your fingers and place 1 heaping tsp. of the egg mixture in the center. Fold the edges so that they meet in the center like a purse and pinch to seal; form each into an oval shape and place on a baking sheet, seam side down. Repeat with remaining dough and filling, rerolling the scraps until they are all used up. Loosely cover pirozhki with plastic wrap and let stand for 20 minutes in a warm place to let rise slightly. Whisk the milk and egg yolk in a small bowl and brush over the pirozhki. Bake until golden brown, 20–25 minutes. Serve warm.