Bring 1 3/4 cups milk, 1/2 cup sugar, zest, and a pinch of salt to a simmer in a 2-quart heavy saucepan over medium heat. In a small bowl, whisk cornstarch and remaining 1/4 cup milk until smooth, then whisk into simmering milk. Bring to a boil, whisking constantly. Boil for one minute. Transfer mixture to a metal bowl, then set bowl in a larger bowl of ice and cold water and cool completely, stirring frequently. Discard zest.