In a saucepan over low heat, melt together the butter, sugar, and cornstarch.
Once melted, add the milk, vanilla, and salt.
Stir and cook until thick.
Once thickened, pour the filling into the cooled pie crust and sprinkle with cinnamon.
Notes:
Make Ahead: You can easily make the crust ahead of time. The pie is also good cool. The crust may get soggy if you make it a couple days before eating.