Did you know that marshmallows have magical invisibility powers? The reason we wait until the very end of the baking time to add marshmallows to a casserole is that if they’re baked slightly over the toasted browning phase, they will disappear! Yes, vanish. There is a minute or so when they are perfectly golden brown, and if you miss that minute—poof!—the ooey-gooey marshmallows explode. This isn’t a bad thing; they will coat the top of these delicious sweet potatoes either way. But try it for yourself and see what you think.