In a heatproof bowl set over (not in) a pan of simmering water, combine chocolate chips and condensed milk. Cook, stirring occasionally, until chocolate melts and mixture is smooth. Pour chocolate mixture over marshmallow topping; working quickly, swirl together with a thin-bladed knife. Refrigerate until set, about 1 hour. Cut into 18 squares. Refrigerate, tightly wrapped in plastic, up to 2 days.