Line an 8-inch square pan with two strips of parchment paper, leaving overhang on opposite sides.
Combine condensed milk and chocolate in a microwave-safe bowl.
Microwave in 30-second bursts, stirring between each, until smooth and fully melted.
Pour the mixture into the prepared pan and spread evenly with a spatula.
Tap the pan gently on the counter to release air bubbles.
Refrigerate until firm, about 2 hours.
Run a knife around the edges, lift the fudge out using the parchment, and slice into 1-inch pieces.