In medium sauce pan, bring milk to simmer over medium heat. Ladle 1/2 cup hot milk into egg mixture and whisk to combine. Repeat with two more 1/2-cup additions, then pour egg mixture back into pan. Cook over medium heat, whisking constantly, until custard turns thick and lumpy, about 3-4 minutes. When custard begins to bubble, continue to whisk over heat exactly 2 minutes. Remove from heat and add vanilla and salt; stir to combine.