Start by lining an 8×8-inch baking pan with aluminum foil. Make sure the foil covers the sides so you can easily lift the fudge out after it sets. This step is crucial for easy removal later on.
In a medium saucepan, combine the 12 oz semi-sweet chocolate chips, 14 oz sweetened condensed milk, and 2 tablespoons butter. Place the saucepan over medium-low heat. Stir the mixture constantly to prevent the chocolate from burning. You want it to melt slowly and become perfectly smooth.
Once melted, remove the saucepan from heat, and let the mixture cool slightly. You don’t want to add the marshmallows while the chocolate is too hot, as they might melt completely and lose their texture.
After the chocolate mixture has cooled for a minute, stir in 1-1/4 cups mini marshmallows and the 1-1/4 cups chopped walnuts. Mix until everything is well combined and the marshmallows are evenly distributed throughout the fudge.
Pour the fudge mixture into the prepared baking pan. Use a spatula to spread it out evenly, ensuring that it reaches the corners of the pan. Tap the pan gently on the countertop to remove any air bubbles.
Now, sprinkle the reserved 1/4 cup mini marshmallows and the 2 tablespoons chopped walnuts on top. Gently press them into the surface of the fudge. This will give the fudge a beautiful finish and extra crunch on top.
Let the fudge sit at room temperature for 30 minutes to cool. Then, place it in the refrigerator for at least 2 hours or overnight to set completely. The longer you let it chill, the firmer it will become.
Once the fudge has set, carefully lift it out of the pan using the foil edges. Peel away the foil, and place the fudge on a cutting board.
Use a large, sharp knife to cut the fudge into squares. Start by cutting it into strips and then cut each strip into pieces. You should get approximately 5 pieces per strip, depending on how large you want them.
Finally, enjoy your homemade Rocky Road Fudge Recipe! Store any leftovers in an airtight container at room temperature, or keep it refrigerated for a longer shelf life.