Line a 9x13-inch baking pan with parchment then spray with cooking spray.
Melt the butter in a large pot over low heat, then add the sweetened condensed milk and stir continuously until fully incorporated.
½ cup salted butter, 14 ounces sweetened condensed milk
Add the caramels to the pot and continue stirring until melted, then add the vanilla and salt.
20 soft caramels, ¾ teaspoon salt, 1 teaspoon vanilla extract
Add 6½ cups of mini marshmallows to the pot, reserving the other 2 cups, and stir until melted.
16 ounces mini marshmallows
Remove from heat and fold in the rice krispies and the remaining 2 cups of mini marshmallows into the marshmallow mixture with a rubber spatula until they are fully coated.
9 cups rice krispies cereal, 16 ounces mini marshmallows
Transfer the rice krispies mixture into the prepared 9x13-inch baking sheet and use another piece of parchment to gently press the rice krispie treats into the pan.
Drizzle caramel sauce and flakey salt on the tops of the rice krispies treats and allow to set for 30 minutes before slicing and enjoying.
Caramel sauce, Flakey salt