No-fail, perfectly domed buttermilk cupcakes with a tender, buttery crumb, covered in 10-minute caramel icing that drips deliciously down the sides. If you don't have buttermilk on hand, add 1 tablespoon of white vinegar or lemon juice to a measuring cup and add enough milk to measure 1½ cups. Stir and let sit at room temperature for 10 minutes.