Mix chili powder, salt, garlic powder, onion powder, black pepper, sugar, mustard, and bay leaf together in a small bowl; season brisket with spice mixture. Arrange beef in a roasting pan or Dutch oven. Do not cover.
Bake in the preheated oven for 1 hour. Fill pan with enough beef stock to cover by 1/2 inch. Cover pan with lid or aluminum foil and reduce heat to 300 degrees F (150 degrees C).
Continue baking until beef is very tender, about 3 hours more. Slice beef thinly, across the grain and serve with juices from the pan.