Heat a pot with plenty of water on high. Peel and slice the sweet potatoes into 2" square pieces or thereabouts. Make sure all pieces are similar in size so they will cook evenly.  Add potatoes to the water and cook over medium-high heat for about 15-20 minutes or until potatoes are tender when pierced with a fork.  Immediately drain potatoes and cool.

While potatoes are cooking, preheat oven to 350° and prepare pie crust.  Brush bottom and sides of crust lightly with one beaten egg white (add yolk and leftover egg white to filling in the next step) to seal the pie crust.  Set aside in refrigerator until filling is ready.

Using an electric mixer on low speed, beat the potatoes until they are light and fluffy.  Add in butter, sugar, remaining egg from previous step + one more egg, milk, vanilla and allspice.  Beat again for 2 to 3 minutes or until the filling is smooth and creamy and well blended.

Pour the filling into the pie shell.  Use a spatula to spread the filling evenly.

Place the pie on the bottom shelf of the hot oven and bake for 45-60 minutes or until the pie is set in the center. (this will take less time with a shallow or smaller pie)  Test with a toothpick.  It should come out clean.   Cool on a wire rack and serve with whipped cream.