Preheat oven to 350 degrees. Line a 8x8 pan with parchment paper.
If you havent done so, you can use a food processor or put your graham crackers into a ziploc bag and beat with something until they are finely turned into crumbs. Combine the crumbs, cinnamon and butter and mix well. Press into the bottom of the pan. Set aside
Let's make the brownies. Melt the butter in the microwave. in your mixing bowl, add melted butter and then add the sugar. Whisk mixture together for two minutes. Add the eggs and continue to mix for another two minutes.
Add the cocoa, flour, vanilla and salt. Combine until fully mixed, being sure to not overmix. Just 1-2 minutes until smooth.
Pour your brownie batter onto the graham cracker rust and spread evenly. Bake for 20-25 minutes.
Let the brownies cool down completely and then set them in the fridge to chill.
Let's make the marshmallows. Combine the gelatin, water, salt and vanilla in your bowl. Stir ingredients lightly and let it sit. It's going to start getting a little hard.
Meanwhile, on your stovetop combine sugar and your corn syrup and another 1/3 cup of water. Stir to combine. Set it over medium heat. This is where you need the thermometer. You want it to reach 250 F. It will take a good 10 minutes.
once you have reached the 250F, turn off the heat and remove. Turn on your mixer where the gelatin mixture is. Slowly pour in the hot sugar little by little. It will take a while but it will eventually turn it into a fluffy white mix. It should take about 10 minutes. Make sure your mixer is on a medium speed.
Once the mixture is thick and spreadable, spread it over the brownies. Cover and place back in the fridge. The gelatin needs to firm up. You can do this for a minimum of 2 hours or overnight.
Once you are ready to serve, turn your broiler on. Make sure your oven tray is placed on the highest level as you want the brownies right under the broiler. Broil for just 1-2 minutes. Keep an eye on it as you don't want them to burn. You want to see a little brown toasty color form but that's it.
To serve, use a large knife. You may want to spray it with non stick spray.