Mix cornstarch, salt and sugar, and blend until smooth with 1 cup of the cold milk.
Scald rest of milk, add the cornstarch mixture, and stir constantly over direct heat until it boils and thickens.
Place over boiling water, cover and cook 10 minutes longer, stirring occasionally.
Remove from heat, stir in butter and vanilla, and pour into large bowl or individual molds (such as custard cups) to cool.
Unmold if desired and serve plain or with any preferred sauce or with preserves or fresh fruit, such as crushed sugared strawberries or sliced bananas.