Line a 9x13-inch baking pan with foil or parchment paper and lightly spray with nonstick cooking spray.
Place the sweetened condensed milk and chocolate chips in a microwave-safe bowl.
Microwave in 30-second intervals, stirring well after each interval, until the chocolate is completely melted and the mixture is smooth. This should take about 90 seconds.
Stir in the peanuts and miniature marshmallows until evenly combined.
Spread the mixture evenly into the prepared pan.
Refrigerate until completely firm.
Lift the fudge from the pan using the foil or parchment paper. Cut into 24 pieces and serve.