Line a 9x13-inch baking dish with parchment paper or aluminum foil. Spray the inside with cooking spray, then set aside.
Add the butter to a large pot and melt over low heat. Once melted, add the sweetened condensed milk and mix until fully incorporated. Then add the caramels to the pot and stir until melted and incorporated, about 5 minutes.
Add the vanilla and salt to the pot and mix well, then ad the mini marshmallows to the pot and stir until fully melted.
Remove the pot from heat. Add the Rice Krispies to the pot and stir until the cereal is well coated in the caramel sauce.
Pour the Rice Krispies treat mixture into the prepared baking dish, then use a spatula to gently press the mixture evenly into the pan. Optionally drizzle on extra caramel or add flaky sea salt.
Let sit for at least 30 minutes to set, then cut into squares and serve.