Add the pork butt, onion, garlic, chipotle peppers, cumin, oregano, bay leaf, chicken broth, and kosher salt to a large 5 or 6 quart slow cooker. Cook on low for 7 to 8 hours, until the pork is tender and falling part.
About 30 minutes prior to serving, preheat a deep skillet over medium-high heat. Remove 5 to 6 cups of the liquid from the slow cooker and add to the hot skillet. Bring the mixture to a rolling boil and cook uncovered until reduced, stirring occasionally (about 25 minutes). If needed, whisk in the cornstarch mixture to thicken the sauce.
While the liquid is reducing, remove the pork from the slow cooker and shred using two forks. Place the shredded pork on a baking sheet and toss with the vegetable oil to coat. Broil on high for 8 to 10 minutes, until the pork starts to get crispy.
Remove the pork from the oven and toss with the reduced liquid. Serve on warm tortillas with diced onions and cilantro.