Line an 8-inch square baking pan with parchment paper, leaving some overhang on the sides. You can also lightly grease the pan with cooking spray.
In a large microwave-safe bowl, combine sweetened condensed milk, unsalted butter, and semi-sweet chocolate chips. Microwave on high for 30 seconds, then stir. Repeat in 30-second intervals until melted and smooth, about 1 to 2 minutes total.
Stir in the vanilla extract once the mixture is smooth.
Pour the fudge mixture into the prepared baking pan and spread it into an even layer.
Refrigerate for at least 2 hours until firm.
Lift the fudge out of the pan using the parchment paper, cut into squares or rectangles, and serve or store in an airtight container.