Line a 9x13-inch baking pan with parchment paper or heavy-duty foil, leaving an overhang on two sides for easy lifting. Lightly butter or spray the lining. Measure out the chocolate chips, milk chocolate, marshmallow creme, vanilla, salt, and walnuts (if using) so they’re ready to go.

If using nuts, spread the walnuts on a baking sheet and toast in a 350°F (175°C) oven for 6–8 minutes, until fragrant. Let cool completely, then roughly chop.

In a large heavy-bottomed saucepan (at least 3 quarts), add the granulated sugar, unsalted butter, and evaporated milk. Stir gently over medium heat until the butter melts and the sugar begins to dissolve, scraping around the edges so no sugar crystals cling to the sides.

Increase the heat to medium-high and bring the mixture to a full rolling boil, stirring constantly. Attach a candy thermometer to the side of the pan, making sure the tip is submerged in the mixture but not touching the bottom.

Cook, stirring frequently, until the mixture reaches 234–238°F (soft-ball stage), about 4–6 minutes after it begins boiling. If you don’t have a thermometer, cook until the mixture thickens, bubbles loudly, and a spoon dragged across the bottom briefly reveals the pan before the mixture covers it again.

Remove the pan from the heat. Immediately add the semi-sweet chocolate chips and milk chocolate. Let sit for 30 seconds, then stir slowly and steadily until completely melted and the mixture is glossy and smooth.

Add the marshmallow creme and stir until fully incorporated and no white streaks remain, 1–2 minutes. Stir in the vanilla extract and fine sea salt until evenly combined.

If making walnut fudge, gently fold in the cooled, chopped walnuts just until evenly distributed, without over-stirring.

Quickly pour the fudge mixture into the prepared 9x13-inch pan. Use an offset spatula or the back of a buttered spoon to spread it into the corners and smooth the top before it begins to set.

Allow the fudge to cool at room temperature until firm, about 3–4 hours. Alternatively, refrigerate for 1–2 hours, though room-temperature cooling gives the best texture.

Once fully set, lift the fudge out of the pan using the parchment or foil overhang. Use a long, sharp knife to cut into 1–1 1/2-inch squares, wiping the blade with a warm, damp cloth between cuts for clean edges.

Arrange the fudge on a serving platter, pack into tins or gift boxes, or transfer to an airtight container for storage. Enjoy at room temperature for the creamiest texture.