Preheat the oven to 300 degrees.
Combine 1/2 cup of the sugar with the salt, cinnamon, cornstarch, and cocoa in the top of a double boiler.
Blend in the cold milk.
Stir in hot milk and cook over hot water until thick, about three to five minutes, stirring constantly.
Cover and continue cooking until very thick, about eight minutes, stirring occasionally.
Add the butter.
Beat the egg yolks lightly and blend them with the remaining 1/2 cup sugar.
Add a little of the hot mixture, then stir into the remaining hot mixture.
Cook, uncovered, over hot (not boiling) water until very thick, about ten minutes.
Add the vanilla. Cool.
Turn the mixture into the baked pie shell.
Top with meringue and bake twenty minutes.
For meringue, combine egg whites with salt and beat until the whites stand in soft peaks.
Beat in sugar one tablespoon at a time, until stiff.