Add the chili powder, garlic powder, smoked paprika, cumin, mustard powder, thyme, brown sugar, salt, and pepper to a bowl and mix until well combined. 

Sprinkle the spice mixture evenly over the pork and massage the spices into the pork. Be sure the spices are covering the pork. 

Heat a large cast iron skillet over high heat and add olive oil. 

When the olive oil is fragrant add the pork to the skillet. Brown for 2-4 minutes on each side (8-10 minutes in total). 

Meanwhile, add the onion, garlic, apple cider vinegar, barbecue sauce, bourbon, and Worcestershire to a bowl and mix until combined. Set aside. 

When the pork is browned, place the pork into the slow cooker and pour the barbecue sauce over the pork. 

Set the slow cooker to low heat and cook for 6-8 hours or until the pork shreds easily with a fork. 

When the pork is cooked, remove it from the slow cooker and place it into a casserole dish or a plate with edges and shred the pork. Remove the bone (if using a bone-in pork shoulder) and any excess fat and discard. 

The amount of sauce left in the slow cooker varies by what type of pork shoulder is used. About 1-1.5 cups of the barbecue sauce mixture is ideal to mix with the shredded pork. You can also add more bbq sauce if you wish.

Add the pork and the sauce that is being used back into the slow cooker and mix. 

Serve over Hawaiian buns or on a tortilla