Follow the instructions to make the graham cracker crumble and set it aside.
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. Stir in the sweetened condensed milk and salt until smooth.
Add 14 ounces of the mini marshmallows and stir until mostly melted and creamy.
Remove the pot from the heat and stir in the key lime juice and zest until fully combined.
Fold in the Rice Krispies cereal and graham cracker crumble pieces until evenly coated.
Let the mixture cool for about 1 minute, then fold in the remaining 2 ounces of mini marshmallows for gooey marshmallow pockets throughout the treats.
Transfer the mixture to the prepared pan and gently press into an even layer. Do not pack the treats down firmly.
Let cool completely before slicing into squares.