Mix pastry flour and salt, cut in cold butter, and add ice water gradually until dough holds together. Wrap a flattened disc and chill 1 hour.
Roll into a 12-inch circle, fit into the pie plate, and crimp. Chill 20 minutes while heating the oven to 375°F (190°C).
Line the shell with parchment and fill with pie weights. Bake 15 minutes, remove weights and paper, then bake 5 minutes more.
Reduce heat to 350°F (175°C). Whisk pumpkin, brown sugar, eggs, evaporated milk, spice, salt, and vanilla gently until smooth.
Pour filling into the warm shell. Bake on a rimmed sheet for 45–55 minutes, shielding the edge if it begins to brown too much.
The outer filling should be set while a small central circle jiggles gently. A thermometer near the center should reach about 175°F (79°C).
Cool on a rack for about 2 hours, then refrigerate at least 2 hours before serving. Do not leave the custard pie out overnight.
Cut chilled slices with a clean sharp knife. Add whipped cream at the table rather than covering the whole pie during storage.
