Slice off the thick slab of fat (known as the fat cap) along one side of the pork butt. Place the whole butt into the slow cooker insert.
In a small bowl, mix together the espresso powder, chili powder, salt, black pepper, garlic powder, mustard powder, onion powder, paprika, cinnamon and cayenne or red pepper flakes. Rub all over the meat, making sure to cover both ends and get into the nooks and crannies. You might think you don't need all of the rub but keep massaging it all over the meat until it's all used.
Set your slow cooker to high for 5 hours or low for 9.
Once it's done, you can shred the meat one of two ways. You can shred it directly in the slow cooker with two forks. But when you transfer it to a serving bowl or container, you probably won't need all of the sauce. Leave about half behind. Stir to blend the meat with the liquid, taste, and add more liquid if you think it needs it. If you want to serve directly from the slow cooker, leave as is. The liquid will help keep it moist.
Or you can transfer the meat to a bowl, shred it with two forks, and stir in about half the liquid. Taste the meat and add more liquid if it's a bit too dry, no more than 2 tablespoons at a time.
Optional step: if you want crispy bits of meat, spread the shredded pork out onto a baking sheet. Broil for 5-8 minutes, or until some parts are crisp.
Serve is as, with Tequila Pineapple Barbecue Sauce, or sauce of your choice.