Creamy Butterscotch Pie

This is one of the creamiest pies you'll ever eat. The velvety texture and taste of the butter and brown sugar together isn't overpowering, just pure deliciousness in every bite! I shared this recipe with a cafe that now claims it as their best seller.
1-1/2 cups packed brown sugar
4 tablespoons cornstarch
3 tablespoons all-purpose flour
2 cups milk
3 egg yolks
2 tablespoons butter
1 teaspoon vanilla extract
1 (9-inch) pie shell, baked
3 egg whites
1/2 teaspoon cream of tartar
1/2 cup white sugar
1 teaspoon vanilla extract
In a double boiler, whisk together the brown sugar, flour, and cornstarch. Add just enough milk to make a paste. Mix in beaten egg yolks, and then add the remainder of the milk. Cook slowly, stirring constantly; when thick, remove from heat. Add butter or margarine and 1 teaspoon vanilla. Pour the filling into the baked pie shell.
In medium mixing bowl, beat egg whites until stiff. Beat in cream of tartar, white sugar, and 1 teaspoon vanilla. Spread meringue over pie filling. Bake at 375 degrees until meringue peaks turn a golden brown.