Line an 8" x 8" pan with foil, leaving some overhang on each side as handles to lift the fudge out later. Spray the foil lightly with cooking spray.

In a large sauce pan, melt the peanut butter and chocolate chips together over low heat. Stir until all of the chocolate is melted and combined with the peanut butter.

Stir in the marshmallows. You don't want the marshmallows to melt too much, so stir them in just until each marshmallow is coated in the chocolate mixture.

Pour into the prepared pan and spread out evenly. Cover and refrigerate until firm.

Lift fudge out of pan using the foil handles and cut into pieces. Store fudge in the refrigerator.