• Line a 8 X 8 X 2-inch baking pan with parchment paper. Cut slits at the corners to help it fit, and use a large enough piece that it goes all the way up the sides of the pan. (This will help you lift the dessert out of the pan when it's done.)

For the crust

• Use a food processor to grind the cereal into a fine powder. You can also do this by placing the cereal in a large zip-lock bag, and then going over it with a rolling pin.
• In a medium-sized saucepan, over medium-low heat, melt the butter with the marshmallows. Mix in the cocoa powder and salt and stir to blend. Pour in the ground cereal and mix just until all of the cereal is coated. Very quickly add this mixture to the prepared pan and flatten it out to cover the bottom. It will be about ⅛-inch thick. (If it's hard to work with and isn't spreading easily -- which can happen as it cools off -- then you can place the entire mixture between two sheets of plastic wrap and roll it flat with a rolling pin. Then, carefully transfer this to the pan and fix the edges to fit properly.) Set aside.

For the peanut butter filling

• In a medium-sized mixing bowl, mix the peanut butter, powdered sugar, cream and vanilla together until it's very smooth. Set aside.

For the chocolate topping

• In a small saucepan, over low-medium heat, scald the cream with the vanilla. Tiny bubbles will appear on the edges of the pan and it should be very hot, but not boiling. Then pour the cream over the finely chopped chocolate. Wait about 4 minutes and then use a wooden spoon and gently stir to combine. It will take a few minutes of mixing before you see it come together. This is essentially a very thick Ganache. Let this cool to room temperature.

To assemble

• Once the chocolate mixture has cooled to room temperature, evenly spread the peanut butter filling over the prepared crust, making sure there aren't any air gaps. Pour the chocolate mixture on top, using a small off-set spatula to smooth it out.
• Place the dessert in the refrigerator, uncovered, for at least 2 hours. Then let it sit at room temperature until you're ready to serve.
• Use the parchment paper to help you lift it out of the pan, and then cut it into approximately 18, 1-inch, rectangular slices. Sprinkle each serving with Fleur de Sel.