In a large bowl, whisk together the flour, sugar and salt. Work the shortening into the flour mixture with your fingertips until evenly dispersed throughout.
Work the butter into the flour mixture with your fingertips, creating flat dime size pieces.
Mix the ice water and vinegar together. Sprinkle over the flour mixture a little at a time, stirring until dough comes together. You may not need all of the water depending on your climate.
Turn the dough out onto a lightly floured surface. Fold the dough over onto itself a few times. Do not overwork the dough! Form the dough into two disks. Cover with plastic wrap and refrigerate at least one hour or up to overnight.
Pumpkin Filling
In a medium bowl, mix together the pumpkin purée, sugar, egg, heavy cream, cinnamon, ginger and nutmeg until well combined.
Assembly
Preheat oven to 400ºF. (200ºC). Line two baking sheets with parchment paper.
On a lightly floured surface, roll dough to 1/8-inch thickness. Cut dough into 12 5-inch rounds, re-rolling scraps as needed. Place dough rounds on prepared baking sheets.
Place 2 tablespoons of filling on each round, leaving a half-inch border. Apply egg wash to edges of rounds.
Fold the dough over and press the edges together. Crimp the edges with a fork. If any filling oozes out, wipe it up with a paper towel or it will burn.
Brush the pasties with egg wash. Cut a half-inch slit into the top of each pastie. Sprinkle with coarse sugar.
Bake until golden brown, about 18-20 minutes. Transfer to a cooling rack.