Whisk together sugar, flour, and salt; set aside. Put buttermilk and yolks in a small saucepan. Gradually whisk in sugar mixture, and cook over medium heat, whisking constantly, until thick enough to coat the back of a spoon, about 4 minutes. Stir in vanilla, zest, and lemon juice.
Pour cream into a bowl set in an ice-water bath. Let cool completely, stirring occasionally. Refrigerate in an airtight container until ready to use, up to 2 days.