Prepare a 9×13-inch pan by spraying it with butter-flavored cooking spray. Set aside.
In a large pot over low heat, melt the butter. Add the sweetened condensed milk and soft caramels. Stir constantly until the caramels are fully melted and the mixture is smooth.
Add 13 ounces of the mini marshmallows. Continue stirring until the marshmallows are melted and the mixture is creamy.
Remove the pot from the heat, then stir in the vanilla extract and salt until well combined.
Add the Rice Krispies cereal and stir with a rubber spatula until evenly coated.
Let the mixture cool for about 1 minute, then fold in the remaining 2 ounces of mini marshmallows. This gives the treats pockets of gooey marshmallow throughout.
Transfer the mixture to the prepared pan. Gently press it into an even layer using a buttered spatula or lightly greased hands. Do not firmly pack the treats down or they can become dense.
Sprinkle flaky sea salt over the top. Let the treats cool for 1 hour before cutting into squares.