Boil sweet potatoes (whole in skin) for 40 to 50 minutes, or until tender. Transfer cooked potatoes to a colander and run cold water over them. Let them cool before removing the skin (Although my mom never does, she’s a pro.).
Mash sweet potatoes in a bowl. Add butter, and mix well with mixer. (It’s best to add the butter while the sweet potatoes are still hot to make sure the butter completely melts.)
Stir in sugar, milk, eggs, nutmeg, cinnamon, and vanilla. Beat on medium speed until mixture is smooth and free of clumps.
Place a small slice of butter onto the unbaked pie crusts. Place crusts in an oven, preheated to 350°F to melt the butter. (This helps flavor the crust.) Remove the crusts from the oven and roll the melted butter around to evenly coat the crusts.
Pour sweet potato filling into the unbaked pie crusts and bake for 60 minutes or until a toothpick inserted in the center comes out clean.
Add 1/2 teaspoon of lemon extract to the filling mix for a zest of flavor.