Adjust oven rack to middle position and preheat oven to 425°F. Make a foil sling for an 8-inch square baking pan by using 2 long sheets of foil that are 8-inches wide. Lay them perpendicular to each other in the pan with excess hanging over the edges of the pan. Push foil into the corners and up the sides of the pan, smoothing foil so that it flush with the pan. Lightly spray the foil with baking spray.
In the bowl of a food processor, add the crackers and pulse until finely ground, about 30 seconds. Add flour, sugar, cinnamon (if using) and salt and pulse until combined, about 5 pulses. Add butter pieces and pulse until the mixture resembles wet sand, about 20 pulses. Press the mixture into an even layer in the prepared pan. Bake until the crust is set and the edges are well browned, about 12-15 minutes. Remove from the oven and let cool completely. (Cooled crust can be wrapped in plastic wrap and stored at room temperature for up to 2 days).
With the oven heated at 425°F, cover the crust with 5 chocolate bars, breaking bars as necessary to create an even layer (you will have some chocolate left over). Chop the remaining chocolate fine and set aside. Spray an offset spatula with non-stick spray. Using a spatula that has been sprayed with nonstick baking spray, spread marshmallow crème over the chocolate. Sprinkle marshmallows over crème. Bake until edges are set and the marshmallows and puffed and golden brown, about 15 minutes. Transfer the pan to a wire rack and sprinkle the reserved chopped chocolate over. Let bars cool until chocolate is set, at least 4 hours.
When ready to serve, lightly spray chef's knife with non-stick spray. Using foil over hang, gently lift the bars from the pan. (Marshmallows may stick to the foil, so gently use a knife to separate, if necessary.). Cut bars into 16 bars and serve. (Bars can be stored in an airtight container for up to 2 days.). Enjoy!