Melt the butter, marshmallows and salted caramel sauce together in a large pot over low heat.
Add the Rice Krispies and stir together until evenly moistened. Transfer the mixture into the prepared pan and, using oiled hands, press the mixture into an even layer. Set aside.
Melt the chocolate in the microwave on 50% power in 30-second increments, stirring after each, until completely smooth and melted. Pour over the Rice Krispies mixture and spread into an even layer with an offset spatula. Sprinkle fleur de sel over top. Place the pan in the refrigerator to set, about 30 minutes. Cut into squares and serve. Leftovers can be stored in an airtight container at cool room temperature, or in the refrigerator. (If it's particularly warm in your kitchen, the chocolate topping may melt.)