Grease the inside of a microwave-safe bowl with unsalted butter and place marshmallows, vanilla, salt and water in a bowl. Microwave for 60 seconds. Stir and ensure marshmallows are melted, if not zap for another 30 seconds and stir to ensure everything is combined and melted.
Stir in corn syrup and gel food coloring at this time if you are going to color your fondant.
Add mixture to stand mixer along with sifted confectioners sugar. Use dough hook attachment and mix at low speed until everything is combined, then mix at medium speed for about 1 minute. At this point, it's pretty sticky!
Remove from mixer and place on surface sprinkled with confectioners sugar. Use vegetable shortening as needed to keep the dough from sticking as you knead in more confectioners sugar and create a smooth dough.
Wrap ball of dough in plastic wrap and place in a ziplock bag to keep from drying out. Let dough rest for at least an hour before using it.
Once you are ready to use the fondant, have vegetable shortening ready to knead once more as you make your creation. This helps it from being dry and gives it a nice shine. I used a flat surface sprinkled with confectioners sugar to roll out the fondant and cut out my shapes.