Melt marshmallows in microwave on high in 30 second intervals until melted and mix until smooth. Mix in flavoring and corn syrup.
Melt chocolate chips in microwave in 30-second intervals and mix until smooth.
Combine marshmallow and chocolate mixtures thoroughly.
Begin adding powdered sugar one cup at a time, mixing completely after each addition, until fondant is a thick dough. The dough may be "stringy" at this point in time and will NOT look like regular Marshmallow Fondant - this is normal. Turn out onto a smooth surface dusted with additional powdered sugar and kneed until smooth, workable, and no longer sticky, adding more powdered sugar as necessary. Wrap tightly in plastic wrap and let sit overnight to "cure."
To roll out, dust smooth surface with additional powdered sugar and use as needed. Brown coloring may be added to achieve desired "chocolate" hue.