Stir the pumpkin puree, brown sugar, egg yolk, pumpkin pie spice, salt, and flour or cornstarch into a thick, smooth paste that holds its shape on a spoon.
Cut and Fill
Roll the dough and cut 3 to 4-inch rounds. Spoon about 1 teaspoon of filling onto one half of each round, keeping the edges clear. Do not overfill.
Fold, Crimp, and Bake
Brush the edges with egg wash, fold each round into a half-moon, and press the edges with a fork to seal. Cut a small vent slit in each top.
Brush the tops with egg wash and set on a parchment-lined sheet. Bake at 375 degrees F for 18 to 22 minutes, until golden and crisp.
Toss the warm pies in cinnamon sugar and let cool slightly before serving.
Notes
Keep the filling thick: A loose filling leaks and makes the pastry soggy.
Don't overfill: A teaspoon per pie prevents blowouts.
Seal and vent: A fork-crimped edge plus a vent slit lets warmth escape.