Preheat oven to 350°F. Prepare a 9 x 5-inch loaf pan by spraying with cooking spray and/or lightly but thoroughly flouring the bottom (if needed – my pans don't need this).
In a medium bowl, combine buttermilk and Grape-Nuts cereal, stirring so Grape-Nuts are completely moistened. Set aside and let stand for about 10 minutes.