Season the roast with salt and pepper. Brown it in oil in a hot skillet for 4 minutes per side and transfer it to a 6-quart slow cooker.
Cook the onion in the skillet for 5 minutes. Add the garlic, tomato paste, ancho powder and cumin and cook for 1 minute.
Stir in the coffee, broth, brown sugar and Worcestershire, scraping up the browned bits. Pour the liquid around the roast.
Cover and cook on low for 8 hours, until the beef pulls apart easily. Transfer the roast to a board and skim the sauce.
Stir in the cornstarch slurry and cook on high for 10 minutes until thickened. Pull the beef into large pieces and coat lightly with the coffee-chile gravy.