Line an 8x8 inch baking pan with parchment paper or aluminum foil, leaving some overhang for easy removal. Lightly grease with cooking spray or butter.

In a large microwave-safe bowl, combine chocolate chips, sweetened condensed milk, and butter.

Microwave on 50% power for 1 minute. Remove and stir thoroughly.

Continue microwaving in 30-second intervals at 50% power, stirring well after each interval, until mixture is completely smooth and no chocolate chips remain unmelted.

Stir in any optional add-ins you desire, such as vanilla extract, nuts, or other mix-ins.

Working quickly, pour the fudge mixture into the prepared pan and spread evenly with a spatula.

Refrigerate for at least 2 hours or until firmly set.

Once set, lift the fudge out of the pan using the parchment paper overhang and place on a cutting board.

Cut into 1-inch squares with a sharp knife. For clean cuts, wipe the knife between slices.