CRUST:

1. Combine flour, sugar, and salt in a large bowl. Using a pastry cutter cut in shortening until mixture resembles coarse meal. Add in egg, water, and vinegar and gently mix together until dough is formed. Wrap dough in plastic wrap and chill for about 15 minutes.
2. Preheat oven to 350ºF. Lightly spray a 10x15x1-inch jelly roll pan with cooking spray and set aside.
3. Once dough has chilled, roll dough on a generously floured surface into a 12×17-inch rectangle. Using a spatula carefully lift dough and transfer to prepared baking sheet.
4. Press the dough into the bottom and sides of pan. Tuck any excess dough in and pinch edges together as desired.

FILLING:

1. In a medium bowl, mix pumpkin purée, evaporated milk, eggs, sugar, pumpkin pie spice, and salt until smooth and blended. Pour filling into prepared pie crust.
2. Bake for 35-40 minutes until center is set. If edges of crust become too dark, cover the edges with aluminum foil.
3. Allow pie to cool for at least 1 hour. Once cool, cut into squares.