Peel the sweet potatoes and place them into a medium-sized pan. Cover them with cold water, add the salt, and boil for 20-30 minutes or until tender. Scoop the potatoes from the water and place them onto a platter to cool. Save ¾ cup of the cooking water.
When the potatoes are cool enough to handle, slice them.
Preheat the oven to 375°F.
Roll one pie crust large enough to fit into an 8 x 8-inch pan sprayed with cooking spray. Cut off any overhanging dough.
Place the sliced potatoes into the pie crust, evenly spreading them.
In a medium bowl, place the sugars, flour, and cinnamon. Whisk to blend and break up any lumps. Add the potato water, melted butter, and vanilla. Stir. Pour this mixture over the potatoes in the pie crust.
Use the other crust to make a top for the pie. Cut out shapes like leaves or make a traditional lattice crust top. Bake the pie for 40 minutes.
Remove the pie from the oven and pour about half the milk dip over it. Cook for an additional 10 minutes. Pour the rest of the dip over individual servings.
Milk Topping
While the pie bakes, stir until smooth 1 tablespoon of the milk with the cornstarch in a small bowl. Pour this cornstarch mixture into the rest of the milk and stir.
Add the milk, cornstarch mixture, sugar, and cinnamon to a medium-sized saucepan. Stir and bring to a boil to slightly thicken the mixture. Stir constantly.
Remove from the heat and add the vanilla and 2 tablespoons butter. Stir to melt the butter.