Begin by greasing your 9x13-inch baking dish with a bit of butter or lining it with parchment paper for easy removal later.
In a medium saucepan over low heat, melt the 3 tablespoons of unsalted butter. Add the entire 10-ounce bag of marshmallows and stir continuously until fully melted.
Pour in the 1 cup of salted caramel sauce and stir well to combine.
Remove from heat and add the 7 cups of Rice Krispies cereal, gently folding until evenly coated.
Transfer the mixture into the prepared baking dish, spreading it evenly and pressing down gently.
In a microwave-safe bowl, melt the 12 ounces of dark chocolate in 30-second intervals, stirring until smooth.
Pour the melted chocolate over the Rice Krispies mixture, spreading it evenly.
Sprinkle a generous pinch of Fleur de sel over the melted chocolate.
Allow the treats to cool at room temperature for at least 1 hour or refrigerate for about 30 minutes.
Once set, cut into squares or rectangles and serve.
