Melt marshmallows in a large microwave-safe bowl in 30-second intervals, stirring after each melting, 1 to 2 minutes. Mix in light corn syrup and coffee-flavored extract.

Melt chocolate chips in a microwave-safe bowl in 30-second intervals, stirring after each melting, 1 to 2 minutes. Fold into the marshmallow mixture.

Stir confectioners' sugar into the chocolate-marshmallow mixture, 1 cup at a time, until a thick, stringy dough forms.

Dust a flat work surface with confectioners' sugar; turn dough out and knead until smooth and no longer sticky. Wrap tightly in plastic wrap. Let fondant rest at room temperature, 8 hours to overnight.

Dust a flat work surface with confectioners' sugar and roll out fondant to desired size.