Make the dough:
- In large bowl, combine flour, sugar, salt, and butter. Transfer to food processor and pulse a few times, about 4 seconds per pulse, until mixture is the texture of coarse meal.
- Return mixture to bowl, add buttermilk and vinegar, and use your hands to work liquids into a crumbly mixture. As dough begins to form, fold it over itself. (This process will create layers, ensuring a flaky crust.)
- Once dough has come together, transfer it to a floured surface and divide it in half. Wrap each half tightly in plastic and refrigerate for at least 1 hour, or overnight.
Make the filling:
- Using a paring knife, cut a shallow “x” in the bottom of each plum, then and blanch in hot water for 2 minutes. Transfer plums to an ice bath. When plums have cooled, peel them and cut each in half to remove the pit.
- In a saucepan, combine all ingredients and cook on low, stirring often to avoid burning. Cook until plums have broken down and mixture is thick, about 12 to 15 minutes.
- Process preserves in four pint jars, or use immediately to fill hand pies.
Assemble the hand pies:
- On a lightly floured surface, roll out pie dough to ¼-inch thick. Working quickly, use 4½-inch ring mold to cut out circles. Discard leftover dough and refrigerate circles for 10 minutes.
- Meanwhile, beat eggs for an egg wash and set aside.
- Working in batches, remove three to four circles at a time from refrigerator. Brush edges of dough lightly with egg wash. Place 1 to 2 tablespoons of filling in each center and fold. Press down around edges thoroughly, sealing with fingers or a fork. Transfer finished hand pies to refrigerator while working on remaining circles.
- In a cast-iron skillet fitted with deep fry thermometer, bring 3 inches of canola oil or lard to 350 degrees. Fry hand pies until golden brown. (Alternately, you can bake hand pies in a 350-degree oven for 14 minutes. Brush tops with more egg wash before they go in the oven.)