Start by greasing your 9x13-inch baking dish with a little butter or lining it with parchment paper for easy removal later.
In a large saucepan over medium heat, melt the 3 tablespoons of unsalted butter. Once melted, add the 10-ounce bag of marshmallows. Stir continuously until the marshmallows are completely melted and smooth.
Remove the saucepan from the heat and stir in the ¾ cup of salted caramel sauce until fully combined with the melted marshmallows.
Gently fold in the 7 cups of Rice Krispies cereal, ensuring every piece is coated in the marshmallow mixture. Be careful not to crush the cereal; you want to maintain that delightful crunch.
Pour the mixture into the prepared baking dish. Use a spatula or your hands (lightly greased) to press the mixture evenly into the dish. Don’t pack it too tightly; you want it to remain light and airy.
In a microwave-safe bowl, melt the 12 ounces of finely chopped dark chocolate in 30-second intervals, stirring in between, until smooth and completely melted.
Once the treats have cooled for about 10-15 minutes, drizzle the melted dark chocolate over the top. You can use a spoon or a piping bag for more control.
Before the chocolate sets, sprinkle Fleur de sel over the top to enhance the flavors with a touch of salt.
Allow the treats to cool completely before cutting them into squares. For best results, let them sit for at least an hour.