1. Combine 1/4 cup whole milk and cornstarch in a bowl, stirring well with a whisk. Combine remaining 3/4 cup whole milk and 2% milk in a medium saucepan over medium heat, and bring to a simmer.
2. Add 1 cup brown sugar, vanilla extract, salt, and 2 egg yolks to cornstarch mixture, and stir well with a whisk. Gradually add warm milk to the sugar mixture, stirring constantly with a whisk. Pour the mixture into saucepan, and cook over medium heat until mixture begins to boil, stirring constantly with a whisk. Cook for 1 minute or until mixture is thick, stirring constantly. Remove from heat. Add 1 tablespoon butter, and stir until butter melts.
3. Place the pan in a large ice-filled bowl for 6 minutes or until the mixture cools, and stir occasionally. Cover surface of pudding with plastic wrap. Chill at least 3 hours before serving.