Begin by preheating the oven to 350 degrees F.
In a medium bowl, mix the graham cracker crumbs with the sugar. Stir in the butter until the mixture resembles wet sand. Press the mixture into the bottom and sides of a 9-inch pie dish to create an even layer. Bake for 10 minutes. Remove from the oven and allow to cool.
In a medium saucepan, add the milk, egg yolks, sugar, and corn starch. While whisking frequently to prevent the mixture from burning, cook on medium heat until the mixture thickens and just starts to boil. Remove from heat and stir in the vanilla extract and butter. Whisk to combine. Pour this mixture into the cooled pie crust.
Add the egg whites to the bowl of a stand mixer fitted with a whisk attachment. Beat on high until the egg whites start to foam. Add the cream of tartar. Then slowly pour the sugar into the egg whites while mixing. Continue beating until stiff peaks form.
Spread the meringue over the hot custard pudding. If desired, use a spatula to create "peaks" in the meringue by slightly pressing the spatula into the meringue and pulling it out quickly. Repeat across the top of the pie.
Place the pie back into the 350-degree-F oven. Bake for an additional 15-20 minutes or until the meringue is golden brown.
Allow the pie to cool and set before serving.