16 ozs marshmallows
2 Tbsp water
Food coloring as desired
1 lb or more confectioner's sugar
1/2 c vegetable shortening
cornstarch if desired
Heat marshmallows and water until melted. The book suggests using a microwave, but to avoid having to transfer, I suggest a double boiler. Stir marshmallows to hasten the melting process. Stir in food coloring as needed, remembering that your color will lighten when sugar is added. Sift 3 cups confectioner's sugar over the top; stir to combine.
Dust a work surface with powdered sugar. Generously grease your hands with shortening. Knead the fondant mixture, adding sugar as needed until fondant is smooth and malleable. Regrease hands and dust surface as needed. Knead until fondant is no longer sticky, is firm yet elastic, and does not rip. You can test this by rolling out a 1/4"-thick piece and placing it over a measuring cup. If it tears, add a bit of water and continue to knead, 8-10 minutes total.
Seal fondant in a zip-top bag, pressing out as much air as possible. Let sit overnight at room temperature. To use, roll out 1/4" thick on a powdered-sugar-dusted work surface.