Preheat oven to 350°F
Lightly dust a large cutting board or flat surface with flour. Roll out a room temperature pie crust sheet, so that it becomes thin enough to cut off about 30%. Your final crust will be approx 5 oz.
Place into a 9-inch pie dish, cutting off the excess dough.
Place pumpkin in a large bowl, Add butter and mix well.
Using an electric mixer, mix in brown sugar, milk, eggs, egg whites, pumpkin pie spice, cinnamon, nutmeg, and vanilla. Beat on medium speed until the mixture is smooth.
Pour filling into unbaked pie crust. Bake about 70-75 minutes, or until knife inserted in center comes out clean.
Cut into 10 slices and serve with whipped cream, if desired.