Preheat oven to 425 F degrees. Line a 9x13" baking dish with parchment paper or aluminum foil and set aside.
In a small mixing bowl, combine flour and 1/4 cup sugar. Add cubes of butter and cut into the flour mixture using a pastry cutter or two forks. This can also be done in a food processor by pulsing until the mixture looks like coarse crumbs.
Press crust into the prepared baking dish in an even layer and set aside. To a large mixing bowl, add pumpkin, eggs, salt, pumpkin pie spice, and 2 cups of sugar. Whisk or beat together. Add evaporated milk and stir until well combined.
Pour mixture over crust in prepared pan and bake at 425 F degrees for 15 minutes. Leave the pan in the oven and reduce oven temperature to 350 and continue baking another 50-55 minutes, until filling is set.
Remove pan from oven and cool completely on a wire rack. Cover and refrigerate at least 2 hours before cutting into bars. I usually make my bars the night before, so they're refrigerated overnight.
Slice into squares and serve with whipped cream and a light dusting of cinnamon if desired.