In a large microwave-safe bowl, combine sweetened condensed milk, butter, and 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 the vanilla.
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. Variations:
Add Nuts: Stir ½ cup of chopped pecans or walnuts into the chocolate mixture when you stir in the vanilla extract.
White Chocolate Fudge: Replace the semi-sweet chocolate chips with white chocolate chips. Just be sure to use quality white chocolate with cocoa butter listed in the ingredients for the best flavor. White chocolate fudge is delicious with ⅓ cup each of chopped pistachios and dried cranberries folded into the fudge mixture as well.
Marbled Fudge: Make half the recipe using white chocolate chips and the other half with semi-sweet chocolate chips. Layer the semi-sweet chocolate layer first in the pan and then spoon the white chocolate fudge over the semi-sweet layer. Use a butter knife to swirl the two flavors together.
Add Marshmallows: Gently fold in 1 cup of mini marshmallows to your chocolate mixture before transferring the prepared pan.