Spray a 9x13-inch baking pan with nonstick cooking spray.
Make one aluminum foil sling by cutting one piece of foil 18 inches long and folding it 8 inches wide. Fit the foil into the length of the pan, pushing it against the sides and corners of the pan. The excess should hang over about 2 inches on the sides.
Cut another piece of foil 14 inches long (if using extra-large foil, fold it to a 12-inch width) and fit it, perpendicular, over the first sheet. Again, the foil should have an excess of a couple of inches on each end.
Spray the foil pieces with nonstick cooking spray and spray a rubber spatula with nonstick cooking spray.
Combine all candy in a medium-size mixing bowl. Mix well.
10 mini Twix candy bars, 10 mini Snickers candy bars, ¾ cup mini M&M’s, ¾ cup mini Reese’s Peanut Butter Cups
Melt the butter in a large Dutch oven set over medium-low heat. Add marshmallows, vanilla, and salt. Cook and stir with the sprayed spatula (re-spraying as needed) until the marshmallows are just melted, about 3 minutes.
8 tablespoons unsalted butter, 20 ounces jumbo marshmallows, 2 teaspoons pure vanilla extract, ¼ teaspoon kosher salt
Remove from the heat and stir in cereal and half of the candy mixture. Stir until the candy is fully incorporated.
10 cups Rice Krispies cereal
Spray the spatula or spray your hands and pour the mixture into the prepared pan. Press the Rice Krispie/marshmallow mixture evenly into the pan. Top with the remaining candy and lightly press the candy into the cereal mixture.
Set the Krispie Treats on a cooling rack and let sit for 1 hour to firm. After an hour, lift the Krispies by the foil sling and return to the cooling rack to cool completely. Cut into 24 equal-sized bars.