In a large microwave-safe bowl, combine 1 (14oz can) sweetened condensed milk, 2 tablespoons unsalted butter, and 2 cups semi-sweet chocolate chips.
Microwave in 30-second increments, stirring in between each interval, until the chocolate is fully melted. Once the chocolate is melted, stir in 1 teaspoon vanilla extract.
Pour the mixture into the prepared pan and spread it out with an offset spatula or knife.
Cover the pan with a lid or plastic wrap and refrigerate until the fudge is set, 2-3 hours. I find overnight is best, in terms of flavor and firmness of the fudge.
Once set, remove the fudge, using the parchment paper from the pan cut it into 1-inch pieces.
Store the fudge in an airtight container in the refrigerator for up to 2 weeks.
Notes
Sweetened Condensed Milk: Be sure to use full-fat sweetened condensed milk, not low-fat or fat-free sweetened condensed milk for the best texture. Doubling the Recipe: Double the ingredients and use a 9x13" pan.Additions: Feel free to stir in up to ½ cup of chopped pecans or walnuts, and or 1 cup mini marshmallows into the chocolate mixture when you stir in the vanilla extract.