FOR BUTTERMILK CUSTARD SAUCE: Whisk together the buttermilk, sugar, cornstarch and egg yolks in a heavy 3 quart saucepan. Bring to a boil over medium heat, whisking constantly, and boil 1 minute. Remove from heat, and stir in 1 teaspoon vanilla extract. Serve warm or cold. Store leftovers in an airtight container in refrigerator up to 1 week.