In a medium saucepan, combine the heavy cream, sugar, and corn syrup. Heat over medium, slowly swirling the pan to dissolve the sugar evenly into the cream. Swirling helps prevent the sugar from sticking to the sides of the pan, which could lead to crystallization. Heat until the mixture reaches 240°F, or the hard-ball stage. Use a candy thermometer to monitor the temperature closely. If the mixture begins to rise and bubble too much, briefly remove it from heat until it calms down, then return it to the burner.

Once the sugar mixture reaches 240°F, remove the saucepan from the heat. Immediately stir in the unsweetened baking chocolate. Stir continuously until the chocolate has completely melted and the mixture is smooth and uniform. Be sure to mix well, as any unmelted chocolate can cause lumps in the final fudge. The chocolate will give the fudge its rich cocoa flavor, perfectly balancing the sweetness.

After the chocolate is fully incorporated, let the mixture sit undisturbed to cool to 110°F. Cooling the fudge without stirring is essential to avoid the formation of large sugar crystals, which can make the fudge gritty. This step allows the sugar to stabilize, helping to create a smooth, creamy texture. Use a candy thermometer to check the temperature, and expect the cooling process to take around 20-30 minutes. Make sure the mixture has reached 110°F before moving on, as cooling too quickly or stirring prematurely can disrupt the texture.

While the fudge mixture is cooling, preheat your oven to 350°F. Spread the chopped walnuts evenly on a baking sheet and roast for 7-10 minutes, tossing halfway through to ensure even browning. Roasting the walnuts enhances their flavor, giving them a toasty, rich depth that complements the sweetness of the fudge. Once they’re fragrant and golden, remove them from the oven and let them cool slightly before chopping, if necessary.

Once the fudge mixture has cooled to 110°F, transfer it to a mixing bowl with a paddle attachment. Add the room-temperature butter, vanilla extract, and salt. Begin mixing on low speed. As you mix, air is incorporated into the fudge, which helps give it a creamy, light texture. The mixture will gradually thicken and lose its glossy appearance, turning matte and firm. This process usually takes about 5-8 minutes. Keep an eye on the texture—when it starts to stiffen and become harder to stir, the fudge is ready for the next step.

Once the fudge has thickened and turned matte, gently fold in the roasted walnuts. Be careful not to overmix, as you want the walnuts to be evenly distributed without breaking up the fudge's smooth consistency. The roasted walnuts add a delightful crunch that contrasts with the creamy fudge, bringing out a nutty flavor that enhances the richness of the chocolate.

Line an 8x8-inch baking pan with parchment paper, leaving an overhang on the sides for easy removal later. Pour the fudge mixture into the pan, using a spatula to spread it out evenly and smooth the top. Let the fudge sit at room temperature for 2-3 hours until it’s fully set. Avoid refrigerating during this step, as it can alter the texture of the fudge, making it too firm or grainy.

Once the fudge is firm and fully set, use the parchment paper overhang to lift it out of the pan. Place the block of fudge on a cutting board, and cut it into squares with a sharp knife. For clean edges, you can warm the knife slightly before slicing. Enjoy your creamy, chocolate walnut fudge, and store any leftovers in an airtight container at room temperature for up to two weeks.