Line your baking dish with parchment paper, leaving some overhang for easy removal.
Combine the chocolate chips, butter, and sweetened condensed milk in a microwave-safe bowl.
Microwave the mixture on 50% power for 30 seconds, then stir until smooth. Repeat in 15-second bursts until fully melted and glossy, about 1-2 minutes total.
Stir in the vanilla extract until fragrant and well combined.
Pour the melted mixture into your prepared dish and spread evenly with a spatula. Gently tap the dish to settle the mixture and remove air bubbles.
If desired, sprinkle a pinch of sea salt or chopped nuts on top while the mixture is still warm. Press lightly to embed.
Refrigerate uncovered for at least 2 hours, or until the fudge is firm and set. For quicker results, freeze for about 1 hour.
Once set, lift the fudge out of the dish using the parchment overhang and place on a cutting board.
Cut the fudge into 16 neat squares with a sharp knife, wiping the blade between each cut for clean edges.
Arrange the fudge bars on a plate or store in an airtight container in the fridge. Serve chilled or at room temperature for a soft, melt-in-your-mouth experience.
