In a small bowl, stir together the pumpkin puree, cinnamon, allspice, nutmeg, and sugar until combined. Set aside.
Preheat the oven to 400°F. On a lightly floured surface, roll the pie crust to ¼" thickness. Using a 3" biscuit cutter, cut out 12–15 rounds.
Spoon about ½ teaspoon of filling onto one half of each round, keeping it away from the edges. Dab the edges with a wet finger, then fold the other half over to create a half-moon shape.
Press the edges firmly with a fork to crimp and seal. Using a small sharp knife, cut a lightning bolt vent into the center of each pasty.
Place the pasties on a parchment-lined baking sheet. Whisk together the egg yolk and milk, then brush evenly over each pasty.
Bake for 15–20 minutes until deeply golden. Allow to cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely. Serve at room temperature.