Melt chocolate chips in a microwave-safe glass or ceramic bowl in 15-second intervals, stirring after each interval, 1 to 3 minutes. Stir in peanut butter until smooth. Pour in sweetened condensed milk and stir until well combined. Pour into the prepared pan.
Using the back of a spoon or with a plastic bag over your hand, press the fudge into the pan. Refrigerate until firm, 2 to 4 hours.
Remove fudge from pan and cut into squares. Store in an airtight container.