Toss the brown rice crisps cereal with ¾ teaspoon cinnamon.
In a large pot, melt and brown the butter over medium-low heat. After it foams, it will begin to turn golden brown with a nutty aroma. Stir frequently, watching carefully until it just browns. Be careful not to burn the butter.
Lower the heat and add the marshmallows and vanilla, stirring until all marshmallows are just melted. Remove from heat.
Add the cinnamon-tossed brown rice crisps cereal and salt, and stir quickly to evenly coat.
Working quickly, spread the cereal evenly in the parchment-lined dish.
Mix the remaining 1 teaspoon cinnamon with the sugar and sprinkle over the treats.
Once cool, temper the Mexican chocolate for drizzling.
Fill a small saucepan with a few inches of water and set a heat-proof bowl over the top so a large portion of the bowl is touching the water. Being careful not to get any water in your bowl, add 2 ounces of the dark chocolate. Cook, undisturbed until a little more than half is melted, then stir until the rest melts. Cook for a few minutes longer until the chocolate stings a little when touched on your wrist or just below your lower lip (a tip from The Kitchn!).
Remove the bowl from the heat (again being careful not to get any water in it) and add the reserved 1 ounce of chocolate, cinnamon, and cayenne. Stir until the chocolate melts. Let sit, stirring occasionally, until the chocolate feels just a bit cooler than your body temperature when touched to your wrist.
With a spoon or spatula, drizzle the chocolate over the rice krispie treats.
Let cool until the chocolate sets. Slice and serve.
This nutritional information has been automatically calculated, and as such, may be incomplete or inaccurate. Please reference the specific ingredients you use for the most accurate nutritional information.