Place the marshmallows in a large microwaveable bowl, microwave on HIGH for 30 seconds to 1 minutemail to start melting the marshmallows, and stir until the mixture is smooth. Slowly beat in the icing sugar, a cup at time, until you have a sticky dough. Reserve 1 cup of sugar for kneading. The dough will be very stiff.
Rub your hands thoroughly with the butter and begin kneading get the sticky dough. As you knead, the dough will become workable and pliable.
Turn the dough out onto a working surface dusted with icing sugar and continue kneading until the fondant is smooth and no longer sticky to the touch, 5-10 minutes. Form the fondant into a ball, wrap tightly in plastic and refrigerate overnight.
To use, allow fondant to comel to room temperature, and roll it out onto a flat surface dusted with icing sugar.