Grease a 9×13-inch baking dish and set it aside.

In a large pot over low heat, combine 3 tablespoons unsalted butter, the entire 10-ounce bag of marshmallows, and ½ cup (113 g) salted caramel sauce. Stir constantly until everything is melted and smooth.

Remove the pot from the heat.

Add 7 cups (196 g) Rice Krispies cereal to the melted mixture and stir gently until the cereal is evenly coated.

Transfer the mixture into the prepared 9×13 pan. Using the bottom of a measuring cup or a spatula, press the mixture into an even, compact layer.

Let the pressed Rice Krispies layer sit for 3–5 minutes so it is not piping hot before adding the chocolate.

Place the 12 ounces (340.2 g) finely chopped dark chocolate in a microwave-safe bowl. Microwave at 50% power in 30-second increments, stirring thoroughly after each increment, until the chocolate is completely melted and smooth.

Pour the melted dark chocolate evenly over the Rice Krispies layer and spread to an even layer with an offset spatula or the back of a spoon.

Sprinkle fleur de sel over the chocolate to taste.

Refrigerate the pan until the chocolate topping is firm, about 30 minutes.

Cut into squares and serve. Store leftovers in an airtight container at cool room temperature or in the refrigerator (if your kitchen is warm, the chocolate topping may soften).