Prepare the watermelon mixture as above, except use 3 cups watermelon, 1-1/2 cups whipping cream, 3/4 cup buttermilk, and 2/3 cup sugar. Transfer to a 9x9x2-inch pan. Cover and freeze for 2 to 3 hours or until almost firm.Break the frozen watermelon mixture into small chunks. Transfer to a chilled mixing bowl. Beat with an electric mixer until smooth but not melted. Return to pan. Cover and freeze until firm. Makes 1-1/4 quarts.