In a saucepan, combine the sugar and corn starch.
Heat over low and stir in the milk, butter and salt.
Continue cooking over low heat, stirring constantly, until thickened.
This takes up to 20 minutes, but it's worth it.
Stir in the vanilla and cinnamon..
Pour into baked pie shell.
Sprinkle more cinnamon over the top if desired.
Cool at least 2 hours in the fridge before serving.