Grease a 9×13-inch baking pan with butter or nonstick spray. For easier removal, line the pan with parchment paper, leaving some overhang as handles, then lightly grease the parchment as well.

In a large, heavy-bottomed pot over low heat, combine the cubed Velveeta and butter. Cook, stirring often with a silicone spatula, until completely melted and smooth, about 8–10 minutes. Keep the heat low to avoid scorching.

Turn off the heat. Whisk in the cocoa powder, vanilla extract, and fine sea salt until no streaks of cocoa remain and the mixture looks like a glossy, dark chocolate sauce.

Add the sifted powdered sugar about 1 cup at a time, stirring well after each addition. The mixture will become very thick; use a sturdy wooden spoon or strong silicone spatula. If needed, you can use an electric hand mixer on low speed, starting very slowly to avoid sugar puffing everywhere.

Once all the powdered sugar is incorporated and the fudge mixture is thick and smooth, fold in the chopped nuts, if using, until evenly distributed. The mixture will be dense, similar to a heavy brownie batter.

Scrape the fudge mixture into the prepared 9×13-inch pan. Use an offset spatula or the back of a spoon to press it into an even layer. If it sticks, lightly butter your fingertips and gently press the surface flat.

Cover the pan loosely with plastic wrap or foil and refrigerate for at least 2 hours, or until the fudge is firm to the touch. For best results, chill overnight.

Once fully chilled, lift the fudge out of the pan using the parchment handles and place on a cutting board. Use a sharp knife to cut into about 1-inch squares, wiping the knife between cuts for neat edges. Serve slightly cool for the best texture.