Combine egg whites and cream of tartar. Beat with an electric mixer till soft peaks form, then set aside
Combine milk and sugar in a medium sauce pan over medium heat until warm
Ladle out about a cup of warmed milk into a bowl with the sifted flour. Whisk smooth and then add to sauce pan with rest of milk.
Add egg yolks and salt to sauce pan, stirring to combine.
Raise heat to medium high, bringing mixture to boil. Stir frequently, until thickened. About 10 minutes.
Remove from heat, add butter and vanilla. Let it sit for 5 minutes to cool.
Fold in egg whites and pour into pie crust.
Sprinkle with cinnamon and bake for 30 minutes at 375 degrees until filling is firm.