In the base of a slow cooker, combine the onion, garlic, ketchup, vinegar, brown sugar, tomato paste, paprika, Worcestershire sauce, chipotle pepper, adobo sauce, cumin, salt, and pepper. Stir to combine.
Add the pork butt pieces and turn to coat in the sauce. Cover and cook on low for 8 hours, until the pork is fork-tender and easy to shred.
Instant Pot version: Cook on high pressure for 40 minutes, then let the pressure release naturally.
Shred the pork directly in the slow cooker using two forks, or transfer to a large bowl and mix with the sauce.
Toast the brioche buns until golden. Pile the pulled pork onto the bottom bun, top with pickles and crispy shallots, then add the top bun. Serve immediately.