Combine the marshmallows and 1 1/2 teaspoons water in a large microwave-safe bowl. Microwave for 30-second intervals, stirring in between each, until melted and smooth, about 1 1/2 minutes.
Add about three-quarters of the sifted confectioners' sugar into the marshmallow mixture and stir to combine. Once the mixture becomes too stiff to stir, grease your counter and hands with shortening (or wear greased plastic, disposable gloves) and dump the mixture onto the counter. Knead, adding the remaining quarter of confectioners' sugar, a little at a time, until the fondant is very smooth, not sticky and holds its shape into a ball without relaxing. Use right away or grease it lightly with additional shortening, wrap tightly in plastic wrap and store in an airtight plastic bag or container at room temperature for up to several days.
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