In a medium saucepan, combine 1 cup of semi-sweet chocolate chips, 1 can of sweetened condensed milk, and 2 tablespoons of melted unsalted butter. Heat over low heat, stirring consistently for about 5–7 minutes until the mixture is smooth and glossy.

Once your chocolate mixture is melted and silky, remove the saucepan from heat. Stir in 1 teaspoon of vanilla extract and a pinch of salt. Mix thoroughly until everything is well incorporated.

Line an 8x8-inch baking dish with parchment paper, then pour the smooth chocolate mixture into the lined dish, spreading it evenly.

Drop spoonfuls of marshmallow fluff over the chocolate fudge layer, then swirl the marshmallow into the fudge using a knife or skewer.

Cover the baking dish with plastic wrap and place it in the refrigerator. Chill the fudge for at least 2 hours.

Once chilled and firm, lift the fudge out using the parchment paper overhang and cut into squares.