Lightly coat an 8-inch-square baking dish with nonstick cooking spray. Place a
10-inch-square piece of parchment paper in the baking dish, coat with nonstick cooking spray, and set the dish aside.
Bring the sugar, evaporated milk, and corn syrup to a boil in a medium saucepan over medium-high heat, stirring occasionally. Reduce the heat to medium and simmer until the syrup reaches 220 degrees F (it will have a foam of finely textured bubbles on top like the foam on a beer), about 8 minutes.
Turn off the heat and whisk in the chocolate, vanilla, and salt until the chocolate is completely melted and the mixture is smooth. Let it cool for 3 minutes (you shouldn't feel any heat if you touch a dab to the bottom of your lip) and then stir in the pecans and marshmallows until combined.
Transfer the mixture to the prepared baking sheet and spread it into a somewhat even layer using a rubber spatula (it's okay if it's a little bumpy and rustic). Tap the pan on your work surface a couple of times to compact, and then cover flush with plastic wrap and refrigerate for at least 1 hour or overnight.
Place a deep bowl or pitcher of hot water next to your work surface. Invert the chocolate onto a cutting board and carefully remove the parchment paper. Dip a chef's knife or pizza wheel into the hot water and cut the bar into 2-inch squares. Place the heavenly hash in paper candy cups, wrap in decorative foil, or arrange on a serving platter and refrigerate. Let the candies sit out at room temperature for at least 30 minutes before serving.