In a small saucepan over medium heat, warm the milk until it is body temperature. Transfer to warm milk to a large bowl and stir in the yeast. Let the mixture sit until the yeast is dissolved and looks cloudy, about 5 minutes. A few bubbles on the surface is also a good sign that the yeast is ready.
Crack one of the eggs in a small bowl and beat with a fork. Add the beaten egg to the milky-yeast mixture along with the flour, sugar, salt, and butter.
Use a wooden spoon to mix everything together until the dough starts to pull away from the sides of the bowl. If after a minute or two of mixing it doesn’t pull away from the bowl, add a little more flour, 1 tablespoon at a time, until it does. If the dough seems too dry and impossible to mix, add a little more milk, 1 tablespoon at a time until it becomes a little more forgiving.
Transfer the dough to a lightly floured work surface and shape the dough into a large ball. Knead it by pressing it with the heel of your hand and pushing it away from you, then immediately pulling it back toward you, folding the dough on top of itself. Turn the dough clockwise a little bit each time you push and pull so that it gets evenly worked and knead until the surface is completely smooth and the whole thing feels both solid and soft at the same time. It may take a solid 5 minutes of kneading.
Put the dough back in the large bowl and cover the bowl with plastic wrap. Let the dough sit in the warmest spot in the kitchen until is soft, puffy, and doubled in volume – about 1 hour.
Return the dough to the lightly floured work surface and use a floured rolling pin to roll it into a large ovalish rectangle measuring roughly 18 x 12 inches. If the dough resists while you are rolling it, simply let it rest until it yields to the rolling pin; dough responds well to to patience. Spread the surface of the dough evenly with raspberry jam, leaving 1/2-inch border.
Starting from the long side, roll the dough up tightly so you end up with a 18-inch rope. Cut the rope into 12 even slices. I like to cut it in half, cut each half in half, and so on and so forth until you have 12 slices. The ends might not have much jam but you can still add them to the bunch.
Line a baking sheet with parchment paper. Arrange the buns, cut side up on the prepared baking sheet in relatively even rows. The buns should be lightly touching each other and the seams face inward towards each other.
Cover loosely with plastic wrap and let rise at room temperature until they’ve risen a bit and puffed, 30 to 45 minutes.
You can also prepare these rolls the night before, refrigerate covered and then bring to room temperature for 1 hour before baking.
Place a rack in the center of the oven and preheat to 350 degrees F.
Crack the remaining egg into a small bowl and whisk it with 1 tablespoon water. Before baking, uncover the buns and brush them lightly with the beaten egg. Bake until the buns are browned and bubbling, 25 to 30 minutes.
While the buns are in the oven, in a small bowl whisk together powdered sugar, sour cream, and vanilla.
Drizzle the warm buns with the sour cream glaze. It’ll be messy. Enjoy it. Serve warm to hot as these buns are best served almost straight out of the oven.