Prepare a 9 x 13 pan:  butter the bottom and sides, then line the bottom with parchment or wax paper, leaving a 4-inch overhang on the ends (so you can lift it out later). Butter the parchment as well.

In a medium bowl, combine chocolate chips, marshmallows, vanilla, and pecans, if using. Set aside.

In a large saucepan, combine evaporated milk, sugar, salt, and butter over medium heat. Stir until sugar and butter are melted, then bring mixture to a boil and cook undisturbed until temperature reaches 218° – 220° on a candy thermometer.*

Remove syrup from heat and pour into chocolate/marshmallow mixture. Working quickly (it thickens as it cools!), combine syrup and chocolate mixture with a rubber spatula or wooden spoon until all ingredients are melted and mixture is glossy and very, very thick. Enjoy thinking about all the calories you’re burning with this major arm workout. Turn into prepared pan, smooth top to even out, and set aside to cool completely at room temperature (at least two hours).

When fudge is cool, run a knife along the inside edges of the pan to release any sticking and lift out parchment onto a flat surface (or, alternatively, flip the pan onto a cutting board and peel off parchment). Cut fudge into 1-inch squares.

Store in airtight containers (with parchment or wax paper in between layers of fudge) at room temperature.