Using a fork, poke the pork butt on all sides, creating a small amount of holes around the meat. Place the pork butt in the slow cooker.
Add the brown ale to the slow cooker, along with a teaspoon of each of the spices (cayenne, cumin, salt, and black pepper).
Cover the slow cooker, then turn it on low for approximately 5 to 6 hours. The meat should be tender when done.
Remove the liquid from the slow cooker, keeping one cup of the liquid to the side for later use.
Pull the pork using two forks, shredding it completely.
Add the sweet BBQ sauce, the cup of liquid, and the remaining cumin, salt, and black pepper. Stir to combine. Taste for spiciness - if more spiciness is desired, add the second teaspoon of cayenne and stir to combine.
Cover again and cook on low for 20 to 30 minutes. Serve on buns or simply on a plate with sides.
If the BBQ sauce is already spicy, you can opt for paprika powder instead of cayenne pepper powder in the recipe.