Line an 8×8-inch or 8×9-inch square baking pan completely with parchment paper or wax paper, leaving an inch of overhang on the sides to act as handles for an easy lift-out release.

In a medium saucepan set over medium-low heat, combine the sweetened condensed milk, semi-sweet chocolate chips, unsalted butter, vanilla extract, and a pinch of kosher salt.

Stir the ingredients continuously with a silicone spatula or heavy wooden spoon as they melt. Keep the burner on low heat to ensure the chocolate melts gently and evenly without scorching or splitting into an oily mess. Cook for 5 to 7 minutes just until the mixture translates into a thick, uniform, glossy, and completely smooth chocolate cream.

If you are choosing to add nuts, remove the saucepan from the hot burner now and fold in the 1 cup of chopped pecans, stirring quickly to distribute them evenly throughout the heavy fudge paste.

Immediately scrape the hot, viscous fudge into your prepared lined pan, using your spatula to firmly press and smooth the top surface into a flat, level sheet. Tap the pan firmly against the counter a few times to flash-release any trapped pocket air bubbles.

Transfer the pan directly into the refrigerator to chill undisturbed for at least 1 hour (or leave it on the counter at room temperature for 3 to 4 hours) until the structure sets entirely bone-firm to the core.

Use the parchment paper handles to lift the solid block of fudge cleanly out of the pan and place it flat onto a cutting board. Peel away the paper lining and use a sharp, heavy chef's knife to cleanly slice the slab into uniform 1-inch squares. Serve immediately or store in an airtight container at room temperature for up to two weeks.