Line a square pan or small dish with parchment paper, leaving enough overhang for easy lifting.
Add the chocolate, sweetened condensed milk, and butter to a medium saucepan.
Place over low heat and stir frequently until the chocolate has completely melted and the mixture is smooth and glossy.
Remove from the heat as soon as everything is evenly combined.
Pour into the prepared pan and spread into an even layer.
Refrigerate for 2–3 hours, or until firm.
Lift the fudge out using the parchment paper.
Cut into small squares with a sharp knife.
Serve chilled or allow the pieces to soften briefly at room temperature.