In a medium saucepan, combine 2 cups of semi-sweet chocolate chips, 1 can of sweetened condensed milk, and ¼ cup of unsalted butter over low heat. Stir frequently for about 5–7 minutes, or until the mixture is completely smooth and glossy, ensuring no chocolate chips remain visible. Remove from heat once melted.

To the melted chocolate mixture, stir in 1 teaspoon of vanilla extract and a pinch of salt, mixing well until fully incorporated. This enhances the flavor profile of your Marshmallow Swirl Fudge. Make sure you scrape down the sides of the saucepan to blend any remaining ingredients into the creamy mixture.

Line an 8x8-inch baking dish with parchment paper, leaving some overhang for easy removal later. Pour the melted chocolate mixture evenly into the prepared dish, using a spatula to smooth the surface. Ensure the layer is even and reaches the corners to achieve a uniform fudge texture when set.

Spoon dollops of 1 cup of marshmallow fluff onto the top of the chocolate mixture, spacing them evenly. Using a knife or skewer, gently swirl the marshmallow into the chocolate without overmixing. You want to maintain prominent ribbons of marshmallow fluff for that delightful contrast when enjoying your Marshmallow Swirl Fudge.

Place the baking dish in the refrigerator and chill for at least 2 hours, or until the fudge is completely set and firm to the touch. The cooling process is essential for getting the perfect texture; the fudge should be sliceable without crumbling when you remove it from the dish.

Once the fudge has chilled and set, carefully lift it out of the dish using the parchment paper overhang. Transfer it to a cutting board and cut into small squares or rectangles to enjoy a taste of your homemade Marshmallow Swirl Fudge. Serve immediately or store in an airtight container for later indulging.