Line 9-by-5-inch loaf pan with strips of waxed or parchment paper; coat evenly with cooking spray. Set aside.
Combine sugar, salt, butter, cream, and marshmallows in a heavy-bottomed medium saucepan. Set over medium heat; cook, stirring, until butter and marshmallows are almost melted, 5 to 6 minutes.
Bring mixture to a boil; cook, stirring occasionally, for 5 minutes. An adult should remove the pan from the heat.
Add chocolate chips and vanilla to saucepan. Stir until chips are melted and combined. If you like, mix in 1/2 cup peppermints or nuts.
An adult should pour hot fudge into the loaf pan. Top with remaining mints or nuts.
Cool fudge in the pan, at room temperature, 3 hours. Grip paper, lift out fudge, and place on cutting board. Discard paper. An adult should cut fudge into squares. Pack in a box lined with waxed paper to avoid sticking.
Instead of using a loaf pan, pour a double batch of fudge into a lined and greased 9-by-13-inch pan; let cool, remove from pan, then cut with cookie cutters.
You can use cookie cutters to delight your loved ones with fudge shaped as snowflake or bite-size "drops." For the snowflakes, dust the fudge with confectioners' sugar.