"The flavor of commercial buttermilk varies enormously from dairy to dairy. Natural fresh buttermilk, if available, would be ideal for this unusual summer luncheon or dessert soup, but cultured buttermilk of good flavor will do almost as well." With an electric beater or wire whisk, beat the egg yolks in a large bowl. Gradually add the sugar, beating until the eggs fall back into the bowl in a lazy ribbon when the beater is lifted. Add the grated lemon rind and juice and the optional vanilla. Slowly beat in the buttermilk, continuing to beat until the soup is smooth. Serve in chilled bowls and float a spoonfull of unsweetened whipped cream on the surface of each service if you like. Buttermilk soup is traditionally served with Oat Cakes. (See recipe.)