Preheat the oven to 170ºF (75ºC) and line a baking sheet with parchment paper.
Place the mini marshmallows on the baking sheet. Spread them out evenly so none of the marshmallows are touching.
Place the baking sheet in the preheated oven and bake the marshmallows for 4 hours or until they are completely dry through the middle. The marshmallows are done when they shatter into powder when tapped with a hammer or other blunt object like a rolling pin. Once completely cool, store the marshmallows in an airtight container with a tight fitting lid for up to one month.
Notes
1 cup of fresh mini marshmallows weighs about 45 grams. 1 cup of dehydrated marshmallows weighs about 35 grams. Take this into consideration when prepping marshmallows for a recipe. You will need to dry extra marshmallows to compensate for the weight of the moisture loss.
You can use large marshmallows and cut them into smaller pieces but it's kind of a pain. Use mini marshmallows if you can.
If you have to use large marshmallows, dip the sticky cut ends in cornstarch to prevent sticking on the baking sheet.
I highly recommend using parchment paper under the marshmallows. Before they dry out completely, they do release moisture and get a bit sticky. Parchment helps reduce sticking to the pan.
The marshmallows absolutely cannot touch one another and need a bit of space around them. They puff up as they dry, and if they are close enough to touch, they will stick together.
Drying marshmallows in the oven changes the shape of the marshmallows. Don't be alarmed if they look a bit deformed, they will still taste great.