Combine sugar, cornstarch, and salt in a pot. Slowly whisk in milk over medium-high heat. Whisk until thick and bubbling, 6 to 8 minutes. Whisking constantly, slowly pour the warm milk mixture into the yolks. Return mixture to pot and cook over medium heat, stirring constantly, until it returns to a boil, 1 to 2 minutes. Remove from heat immediately and strain into a bowl; stir in remaining 2 tbsp. butter and vanilla.