Yuki from The Return loves pink, glitter, colouring books, looks about 13-14ish, and she will fuck you up.
Jade Sinclair (Generator) of the Whateley Universe. Cute as a button, looks like she's only eleven... and has scared the holy <censored> out of violent villains like Bloodwolf and Killstench. Bloodwolf controls a werewolf spirit and likes hurting people. He got stapled to a tree with railroad spikes. Do not make Generator mad at you. And then there's the Syndicate hardsite she destroyed... after being tortured nearly to death, stabbed through the heart, then gives herself CPR and plays dead until her regen kicks in, then she goes about killing half the base herself with jagged metal objects and raising her victims as "zombies". It starts to cross the line into Grotesque Cute, really.
In Dave Madson's Looney Tunes Intro Bloopers, Microsoft Anna is portrayed as a beautiful 25-ish year old woman with laser eye powers that are often used to defeat logos that scare Microsoft Sam.
Rebecca Stone from Demo Reel. Sweet and pretty, but will slam your head into something hard if you're sexist to her or just outright beat you to death if you hurt her friends.
Ruby Rose from RWBY is a deadly fighter who can effortlessly slaughter her way through hordes of monsters using Crescent Rose, a sniper rifle/scythe hybrid that is known as one of the most dangerous weapons ever designed. That said, whenever she opens her mouth, especially when she's frustrated or excited, she's a sweet little girl who is as adorable as a box of kittens.
From the same series, Nora Valkyrie. She's bright, peppy and cheerful with just about everything she does, including pounding Grimm to death with a grenade launcher-hammer hybrid.
Every female character in RWBY is this trope to a lesser extent. Jaune as well. It could be said that RWBY is not just a World of Badass, but a World of Badass Adorable.
The absolute king of this trope in RWBY, however? Zwei, Ruby and Yang's pet corgi. In the finale of Volume 2, he participates in the battle on the train and holds the line with Oobleck against the White Fang Paladins, even allowing the teacher to use him in a fiery Fastball Special. After the train crashes, Zwei makes a grand entrance by headbutting a Beowolf◊.
Velvet Scarlatina. A rabbit-eared girl shown to be very timid and shy, but she also cares deeply about the safety and wellbeing of others. But then she reveals her abilities.... or rather, everyone else's. By photographing people with her camera, she can create hard light copies of other people's weapons and use them with the exact same level of skill as the original wielders. Case in point? She takes down a huge paladin all by herself.
Bee from Bee and Puppycat. It's a rare Magical Girl that will immediately lunge at a monster and then start attacking it with her teeth. And win.
Puppycat also qualifies, thanks to his ability to incinerate that same monster, but might take issue with being called adorable.
The Angry Video Game Nerd's pet cat, Death Kitty (real name: Boo) chased away the Atari Jaguar logo when it invaded the Nerd's room and proved impervious to his light guns. As the Nerd himself puts it, Death Kitty is "the real Jaguar".
Flotsam the adorable bomb filly from Fallout Is Dragons. She serves as the team's moral core... and has no qualms about throwing gernades at her enemies, bowling for undead with explosives, or mining an entire dune to wipe out a raider encampment. Her best friend is a necromancer and together they pioneered the concept of Bombies, zombies loaded with explosives and sent after their foes.