Preheat smoker to 275°F. Cut chuck roast into 3 inch pieces. Rub pieces with mustard and season with Rancher Rub. Smoke for 3 hours.
Transfer smoked chuck roast to a metal pan. Add pads of butter, cola (or beer), and Smoky Maple BBQ Sauce. Cover the pan with foil and back to the smoker for another 2 ½ - 3 hours or until roast is tender and shreds very easily. Remove the pan from the smoker and rest until cooled slightly.
Shred meat to desired texture and toss with the liquid in the pan. Set aside.
Prepare corn salsa by combining corn, bell pepper, red onion, cilantro, mayo, cotija cheese, and lime juice.
Serve shredded beef on tortillas with corn salsa and other desired toppings.
