I can do it with ease.
A little graphic
There's a trick to the challenge, so it's fun to bet money (I made $50 once) and watch reactions. Just before you're about to do the challenge, show you mouth wide open (this is standard with anyone, to make sure you don't have any liquids to cheat with). Then, all you do is press your tongue into your teeth as hard as you can. Your mouth will start producing a lot of saliva. You get a good amount in there before you take the spoon of cinnamon. Then press the walls of your mouth together, and lay your tongue flat, so all the saliva is on top of your tongue.
Then when you put the spoon in your mouth, flip the spoon over inside and flatten your tongue. Press your tongue up as hard as you can against the spoon while you're dragging it out. It makes an easy to swallow paste. People think you're just flipping the spoon and licking it clean, dramatic effect lol. "OHHHHH, HE TOOK EVERY LAST SPECK" "OHHHHHHHHHHHHHHHH". It still sucks, but it's 9001x easier than trying to do it "normally".
Easy-sauce.