Place the marshmallows in a large microwave-safe bowl. Microwave on high for 1 minute, until the marshmallows are puffy and expanded.2. Use a rubber spatula and stir until they are melted and smooth. If some unmelted marshmallow pieces remain, return to the microwave for 10 seconds, until the marshmallow mixture is entirely smooth and free of lumps. If you want colored or flavored fondant, you can add several drops of food coloring or extracts at this point and stir until incorporated (cotton candy extract is a fun and tasty one to try).3. Add the powdered sugar and begin to stir with the spatula. Stir until the sugar begins to incorporate and it becomes impossible to stir anymore.4. Scrape the marshmallow-sugar mixture out onto the prepared work surface. It will be sticky and lumpy, with lots of sugar that has not been incorporated yet--this is normal. Dust your hands with powdered sugar (or coat with oil/butter), and begin to knead the fondant mixture like bread dough, working the su