4 cups powdered sugar, plus extra for rolling/kneading
Shortening for greasing parchment and hands
Add your mini marshmallows and 2 tbsps of water to a large, microwaveable bowl. Microwave on high for 1 minute (they will rise and expand, so keep an eye on them! ) Alternatively, add marshmallows and water to a large pot and melt on medium heat on the stovetop.
Using a spatula or wooden spoon, mix your marshmallows until completely smooth. If marshmallow chunks remain, heat for another 30 seconds in the microwave. (Optional: Once your marshmallows are melted completely, add in food colouring and/or candy flavouring and combine.)
Add your icing sugar, one cup at a time and combine. Mixing your melted marshmallow with icing sugar will be a great workout for your arm.
I don't recommend using a mixer for this step - the fondant gets really sticky, and will be really hard to clean!
Once your fondant is totally mixed, it'll be thick, and really sticky. Prepare a large sheet of parchment or waxed paper on the counter.
Use your shortening to grease the parchment or waxed paper, and then sprinkle icing sugar over top. Transfer your fondant to the sheet.
Wrap your fresh fondant up with the waxed/parchment paper, and chill in the fridge for a couple hours so the melted marshmallows can totally firm up again.
Once your fondant is chilled, cover your countertop and hands with icing sugar, and roll/knead the fondant until its no longer super sticky. Store in an airtight container. Before using my marshmallow fondant, I like to grease my hands with a small bit of shortening; this really helps the fondant from sticking to my hands and my molds, and also prevents the fondant from cracking and drying out too quickly.