Lemony Sweet Potato Pie

Yield:	8 Servings
Categories:	Pies, Desserts

3		Egg yolks
1	c	Sugar
1	tb	Flour
1 1/2	c	Milk
1/2	ts	Lemon extract
1	ts	Lemon peel,grated
2	tb	Butter,at room temperature
1	c	Sweet potatoes,mashed,cooked
1		Pie shell,unbaked,9"
		Sweetened whipped cream