Prepare an 8x8-inch baking dish by lining it with parchment paper, allowing extra to hang over the edges for easy removal. Apply a light coating of nonstick cooking spray and set it aside.
In a large microwave-safe bowl, mix together the chocolate chips, sweetened condensed milk, and heavy cream. Microwave in 30-second bursts, stirring between each, until the mixture is melted and smooth. Allow it to sit for 1 to 2 minutes.
Fold in the miniature marshmallows.
Evenly distribute the fudge mixture into the prepared baking dish. Place in the refrigerator for 2–3 hours, or until firm.
Using the parchment overhang, lift the fudge out of the pan. Cut into twenty-four 1½-inch x 1½-inch squares. Store in the refrigerator until ready to serve.