If you cannot locate umami powder (a seasoning made out of powdered mushrooms), simple use 1/2 tablespoon more salt. Togarashi has a very distinct flavor that combines pepper flakes, sesame seeds, seaweed, and other spices. You can simply replace this with cayenne pepper to get a burst of spiciness.
Prepare smoker for a 6 to 8 hour smoke at temperatures around 250 to 275 degrees F.
Combine the rub ingredients and apply to the meat evenly. Chuck roasts have lots of crevices and spaces. Work the rub into these areas but don't leave heavy pockets of the rub.
Place the chuck roast on smoker once it has reached temperature. Cook for 6 to 8 hours or until the meat reaches an internal temperature of 190 degrees Test the meat temperature in several locations in thick chunks of the meat.
Once the meat has reached 190 degrees F remove it from the smoker, wrap in foil and allow to rest for 10 to 20 minutes.
Afterwards, unwrap the roast and shred or cut into small bite-sized pieces.