In a medium saucepan, combine 2 cups of semi-sweet chocolate chips, one 14-ounce can of sweetened condensed milk, and 2 tablespoons of unsalted butter. Set this saucepan over low heat, stirring constantly for about 5–7 minutes, until the mixture is completely smooth and glossy.

Once your chocolate mixture is silky and melted, remove it from the heat. Stir in 1 teaspoon of vanilla extract and a pinch of salt, mixing well until fully combined.

Line an 8x8-inch baking dish with parchment paper, allowing some overhang to make removal easier later.

Pour the smooth chocolate mixture into the prepared dish and use a spatula to evenly spread it out into all corners.

Dollop spoonfuls of 1 cup of marshmallow fluff on top of the chocolate layer. Using a knife, gently swirl the fluff into the fudge without overmixing.

Refrigerate the fudge for at least 2 hours, or until it is firm to the touch.

Once chilled and set, lift the fudge out of the baking dish using the parchment paper. Place it on a cutting board and slice into squares with a sharp knife.