Spray a 9x13-inch baking pan with nonstick cooking spray, then line it with two overlapping sheets of aluminum foil to create a sling with about 2 inches of overhang; spray the foil with nonstick spray.
Lightly spray a rubber spatula with nonstick spray so the marshmallow mixture won't stick.
Cut each mini Twix and Snickers bar into 8 pieces and place all chopped candy (Twix, Snickers, mini M&M's, and halved mini Reese's) in a medium bowl; mix to combine.
In a large Dutch oven over medium-low heat, melt the butter. Add the jumbo marshmallows, vanilla extract, and kosher salt and stir continuously with the sprayed spatula until the marshmallows are just melted, about 3 minutes.
Remove the pot from heat and immediately stir in the Rice Krispies cereal and half of the mixed candy until evenly combined.
Transfer the mixture to the prepared pan, using the sprayed spatula (or sprayed hands) to press it evenly into the pan.
Scatter the remaining candy evenly over the top and press it lightly into the surface so it adheres.
Set the pan on a cooling rack and let the treats sit for 1 hour to firm; then lift the treats using the foil sling and return them to the rack to cool completely before cutting into bars (12–24 pieces).