Bake pie crust according to package instructions. Cool completely.
In a heat proof bowl, whisk together the eggs, sugar, cornstarch, flour and vanilla. Set aside.
In a medium size saucepan, heat the milk and butter at medium setting. Do not bring to a rolling boil.
Add about a fourth of the milk to the egg mixture, whisking to incorporate it evenly. Pour the mixture back into the saucepan with the remainder of the milk. Return to heat and stir constantly until it thickens. Do not boil!
Pour the filling into the pie crust and let cool completely. Refrigerate until custard is solid. Sprinkle with cinnamon before serving!