You may remember grabbing a bag of marshmallows from the snack cupboard when you were a kid, only to find your brother had left the bag opened and the marshmallows had all become hard little rocks. That inadvertent "forgot to seal the bag" approach is still the most foolproof way to harden marshmallows -- exposing them to the air -- but is not necessarily the fastest way to do it. A combination of a short exposure to hot air, followed by a rapid cool-down, is the next best thing when you need to harden marshmallows quickly.
Microwave Marshmallows
Place marshmallows on a microwave-safe plate, arranged in a single layer with space between each one.
Put the plate in the microwave and heat the marshmallows on high for 60 to 90 seconds. You should see them increase in size while they heat up.
Remove the marshmallows from the microwave and let them sit for five minutes. They will shrink back down to their original size and will harden during that time.
Oven Method
Preheat oven to 350 degrees Fahrenheit.
Arrange marshmallows in a single layer on a clean baking sheet, leaving space between them.
Place the baking sheet in the oven, close the door and turn off the heat. Leave the marshmallows in the oven for 10 to 15 minutes.
Take the baking sheet out of the oven and let it sit out in the air until the marshmallows are cool and hard. You can also put them in the refrigerator to speed their cooling and hardening.