Make the graham cracker crust: In a large bowl, combine graham cracker crumbs, granulated sugar, and melted butter. Mix together until the crumbs are well moistened with the butter, and it starts to stick together.
Pour the crumb mixture into the base of a 9 inch pie plate or tart pan. Firmly pack the crumbs into the bottom and up the sides of the pan. It's important to press it in as firmly as you can to prevent crumbling! Pop the crust into your fridge to chill while you prepare the filling.
Make the pastry filling: Place butter and vanilla into a heat safe bowl, and set aside.
Pour your milk into a large saucepan. Set the pan over medium-high heat, and warm the milk until it starts to bubble along the edges. Set aside to prep the egg mixture.
In a large bowl, combine egg yolks, sugar, and cornstarch. Whisk together until combined and smooth.
Gradually pour the hot milk into the egg mixture. Make sure to move slowly and whisk well after each addition to prevent cooking the eggs!
Strain the milk and egg mixture back into a large saucepan. Set the pan over medium heat, and cook, whisking constantly, until it boils and thickens enough to coat the back of a spoon.
Pour the hot custard into your heat safe bowl with the butter and vanilla. Whisk until the butter is melted and you're left with a smooth mixture.
Cover the custard with plastic wrap touching the surface of the custard (to prevent a skin from forming). Set aside to cool completely to room temperature.
Assemble your tart: Pour the cooled custard into your graham cracker crust, and smoothen the top. Cover and refrigerate for 4 hours, or until the custard is set enough to slice. (Optional) Top the tart with whipped cream and fresh fruit before slicing and serving.