1-In a bowl, whisk together 1/3 cup buttermilk, 1/2 tablespoon sour cream, 1 cup warm water, 1 large egg, and 1 teaspoon salt until everything blends smoothly.

2-Using a dough hook, add 2 cups of the 3 1/2 cups plus 3 tablespoons all-purpose unbleached flour and mix on low speed until incorporated.

3-Gradually add the remaining flour, 1/2 cup at a time, mixing well after each addition. Keep going for about 15 minutes, then add flour 1 tablespoon at a time until the dough no longer sticks to the bowl sides.

4-Mix for an additional 5 minutes, making the total mixing time 20-25 minutes. The dough should end up elastic and slightly sticky but not sticking to your fingers. Cover it with plastic wrap and set it aside.

5-Divide the dough into pieces about the size of gum balls. Roll each one into a 3-inch flat circle on a floured surface.

6-Place two thinly sliced plums on half of each circle, avoiding any juice on the edges, and sprinkle 1/2 teaspoon sugar over the plums.

7-Fold the dough over the filling and pinch the edges tightly to seal. Lay the vareniki on a floured board.

8-Bring 4 quarts of water to a boil with 3/4 tablespoon salt. Cook the vareniki in batches, adding a few at a time. Once they float, cook for 1 more minute, then remove with a slotted spoon and drain well.

9-Sprinkle sugar between layers to keep them from sticking and to make a light syrup. Serve warm with sour cream or on their own.