Combine sugar and flour in a large bowl; add eggs and buttermilk, blend with a mixer. Blend in butter and vanilla extract, and pour into unbaked pastry shell.
Bake at 350 degrees F for 45 minutes or until set.
Check for doneness after the minimum baking time by gently shaking the pie. The center should be set with a slight jiggle.n Do not overcook, or the pie will have the consistency of an omelet instead of a custard.
Cool on a wire rack for 10 minutes.
Melt the chocolate, pour over the pie and spread evenly. Cool in freezer for about 45 minutes to set chocolate.
Instead of buttermilk, you can use cow, soy almond, or coconut milk and two tablespoons buttermilk powder.