Preheat the oven to 350˚F. Generously spray a 12x18-inch sheet pan with baking spray OR rub with shortening then add a tablespoon of flour and tilt the pan in all directions to coat the shortening with flour.
Add 4 tablespoons of the buttermilk (or half and half or milk) and vanilla. Whisk well to combine. Add 3½ cups of powdered sugar and whisk until smooth. You’re looking for a fairly thick but still pourable icing. If too thick, add a bit more buttermilk. If too thin, add a little more powdered sugar and stir again until nice and smooth.
See Café Tips above for more detailed instructions and tips.
Cake recipe adapted from My Recipes, icing, a Café concoction.