In a large sauce pan, not over heat, combine the sugar, flour, cornstarch, and salt. Gradually add the milk until it's completely combined and smooth. Cook over medium heat until mixture thickens, then remove from heat.
In a separate bowl whisk together the egg yolks lightly, add a few spoonfulls of the hot mixture from the stove and whisk together till smooth. Add the egg yolk mix to the rest of the hot mixture. Also add the butter and vanilla and thoroughly combine.
Cool the mix completely in the fridge. When ready to eat, pour the mixture into a pre-cooked pie shell, drizzle with butterscotch topping, and sprinkle with butterscotch chips. Optional- top with whipped topping.