Bring the milk to a simmer in a medium saucepan. In a medium bowl, whisk the sugar with 1 tablespoon of the cornstarch. Whisk in 1 egg and the salt. Gradually whisk in the hot milk, then return it to the saucepan and cook over moderate heat, whisking, until thickened and just beginning to bubble around the edges, about 4 minutes. Strain the custard into a bowl and let cool slightly. Whisk in 2 tablespoons of the butter.