Step 1: Preparation. Line an 8×8-inch square baking pan with parchment paper or aluminum foil. Leave an extra inch of overhang on the sides so you can easily lift the fudge out later. Lightly grease the foil with a small amount of butter or non-stick cooking spray.
Step 2: Melting the Chocolate Base. In a large microwave-safe bowl, combine the dark chocolate chips, milk chocolate chips, sweetened condensed milk, and butter. Heat in the microwave in 30-second intervals, stirring well after each burst, until the chocolate is completely melted and the mixture is silky smooth. Alternatively, you can use a double boiler on the stove over low heat.
Step 3: Adding Flavor and Texture. Once the chocolate mixture is smooth, remove it from the heat. Stir in the vanilla extract and salt until fully incorporated. Gently fold in the chopped walnuts using a spatula until they are evenly distributed throughout the fudge base.
Step 4: Creating the Swirl. Pour the chocolate mixture into the prepared pan and smooth the top with a spatula. Melt the white chocolate chips separately in a small bowl. Drizzle the melted white chocolate over the dark base and use a knife or toothpick to gently swirl the colors together for a marbled effect.
Step 5: Chilling and Setting. Place the pan in the refrigerator for at least 3 hours, or until the fudge is completely firm. For the best results, leaving it to set overnight is ideal.
Step 6: Slicing. Use the parchment paper overhang to lift the fudge block out of the pan. Place it on a cutting board and use a sharp knife to cut it into 1-inch squares. For clean edges, wipe the knife clean between each cut.