Any genre, though I'd prefer a comedic element. BL is okay.

I love the trope of the guy who is misunderstood because he looks like a delinquent/stone-cold a-hole but is really a sweetheart.

Some examples: Kitano from Angel Densetsu, Ryuuji from Toradora!, Asuka from Otomen, Legosi from Beastars, Katai from Komi-san wa Komyushou desu, Kasanoda from Ouran High School Host Club, and (to a lesser extent) Tatsu from Gokushufudou: The Way of the House Husband.

I've looked around the TV Tropes page for "Face of a Thug" but I've either already read the title or it's not really descriptive of what I'm looking for. When I try to look for titles on this site, it generally gets tagged under "misunderstood protagonist" or "ex-delinquint/yakuza" which is too broad to really hone in.

Any recommendations? Bonus points if the happier and more excited they get, the scarier they become. It just always makes me laugh.

Junjou Drop Very short romance story about a guy who looks scary and has the reputation of being a delinquent.

Ore Monogatari!! Kinda fits too.

Maybe Horimiya? It's only really at the beginning though.

And you might also enjoy He is a High-school Girl.

Also the shoujo Beast Master has a scary but sweet guy (but he isn't a delinquent).

GTO!
great teacher onizuka!

Curiosity is a cockroach-like thing.

Kawaii Hito (Saitou Ken)
Kuzumi-kun, Kuuki Yometemasu ka? (Haruna, one of the major side characters)
Ookami Shounen wa Kyou mo Uso o Kasaneru
Sentou Jousai Masurawo
Yankee Wa Isekai De Seirei Ni Aisaremasu. maybe

EDIT: Forgot one. Senryuu Shoujo