Powdered buttermilk (mixed with water, per the instructions on the packaging) is a smart substitute, as it stays fresh (refrigerated) for far longer. For neater bars, tease apart the marshmallows, and cut in between then, rather than through them (the marshmallows won't slice very neatly). The texture of these bars is best the day of, but they are still great after a night or two in the fridge. If you have a culinary torch, use it (instead of the broiler) to toast the marshmallows as it's a more controlled process.