Melt chocolate chips in large saucepan over low heat, stirring constantly until smooth. Remove from heat. Stir in frosting. Fold in marshmallows. Let stand 15 minutes or until mixture is cool enough to handle.
Divide mixture in half. Pour mixture onto sheet of plastic wrap, scraping mixture from saucepan. Shape each half into 9x2-inch log. Wrap tightly in waxed paper; refrigerate 2 hours or until firm.
With thin sharp knife, cut log into 1/2-inch-thick slices.