As the main baker of the Tower, Hank was in a good position to unleash a world of pranks on the other Avengers. They all assumed he was too grown up or kind to actually do anything, which is why they felt safe when trying his sugary and sweet treats. Boy, were they wrong!
It was the middle of another spur-of-the-moment prank war that one of the three pranksters had cooked up which erupted into a full-blown war between just about everyone. Otherwise known as a regular Tuesday.
Hank had made his way to the kitchen, almost getting hit by two paintball gunshots, three water balloons, and the odd tossed object or two. While obviously looking annoyed but mostly tired, no-one could've guessed what he had in mind.
He calmly walked into the upstairs kitchen and closed the door behind him, then got to work making sugar cookies. Meticulously following his mother's recipe, he added in the ingredients, but covertly swapped one.
He walked back out holding the tray, and the reactions were immediate. Everyone put down their weapons and followed him to the observatory, one floor above. He set the tray down only after everyone had arrived, and promptly left without bothering to regulate the cookie intake.
Jan passed by and fist bumped him, and while Sam had found this odd at the time, he'd soon realize why.
The unsuspecting victims bunched around the cookies and snatched as many as they could, impulsively taking huge bites or popping them into their mouths the moment they could. Once again, the reaction was instantaneous, and instead of the sugary sweetness they had been expecting, their taste buds were assaulted with an overwhelming salty taste.
Thor simply cringed, but went on chewing and swallowing every bite, unlike Natasha who barely choked it down and looked as if the cookie would make a (un)surprise reappearance.
Steve spit it back out onto a napkin and pitched it on his way to the sink to wash out his mouth. Sam and Wanda followed suit.
Rhodey, upon realizing his terrible mistake, chewed it as fast as he could, then bolted for the sink and downed 8 oz of water before coming back up for air. Tony just went straight for the sink and washed it down, having already chewed it before realizing why it tasted funny.
Banner gagged, but managed to keep it down, then disappeared to his room, where he wasn't seen for the rest of the day.
Clint and Pietro spit it into a napkin, then folded it up, threw it out, and used a second napkin to scrape as much of it off their tongues as possible. Clint banged his head against the wall, regretting the stupid mistake of A) not waiting to watch everyone else's reactions and B) eating the whole thing in one bite.
Carol and Peter, both of whom couldn't tolerate the amount of salt consumed, ran for the bathroom and puked the cookies back up. They also retreated to their rooms for the rest of the day.
The following day, Hank received glares from Natasha, Pietro, and Tony. Steve, Thor, and Sam gave him grim looks, though Hank never deciphered what they meant. Bruce and Peter didn't cross paths with him the next day, as both had stayed in their rooms until dinner.
Wanda and Clint didn't look at him for another two days after the incident, and the first look from Wanda would've made Skull himself flinch.
Carol was the one who proved the most adept and brave at payback, and Hank found himself locked out of his room for a day before someone finally talked her into letting him back in. No-one bothered to notice the all-out prank war was over, and it was almost a full month before anyone dared to start another one.
Hank's treats are no longer tried during said wars, as everyone knows he's willing to try tricks like that again. He also escaped being anyone's intended target for about three months, which was a feat impressive enough that he hung a self-made award on his wall commemorating the record.