Combine milk and salt in a large, heavy saucepan; cook over medium-high heat just until mixture starts to boil. (Be careful not to let milk boil out of the pan as it comes to a boil – it can do so quickly.) Gradually whisk in grits. Reduce heat; simmer, uncovered, 20 to 25 minutes or until thick, stirring often. Remove from heat, and add cheese, butter, and pepper, stirring until cheese melts; cover and keep warm.