In a medium saucepan, combine 2 cups of semi-sweet chocolate chips, 1 can of sweetened condensed milk, and ¼ cup unsalted butter over low heat. Stir until melted into a glossy mixture, about 5-7 minutes.

Remove the saucepan from heat and stir in 1 teaspoon of vanilla extract and a pinch of salt until fully integrated.

Line an 8x8-inch baking dish with parchment paper and pour the fudge mixture into the dish, smoothing it out evenly.

Spoon dollops of 1 cup of marshmallow fluff over the fudge and swirl gently with a knife for a marbled effect.

Refrigerate the fudge for at least 2 hours until firm.

Lift the fudge out using the parchment paper, cut into squares, and serve.