Remove the membrane from the back of the ribs, coat with mustard, and apply the dry rub.
Preheat the pellet grill to 225°F and smoke the ribs, meat-side up, for 3 hours.
Lay each rack on foil with brown sugar, butter, and apple juice; wrap tightly and smoke 2 more hours.
Unwrap, brush both sides with BBQ sauce, and smoke 1 final hour until the glaze sets.
Check for a bend: the rack should crack slightly on the surface when lifted.
Rest 10 minutes, then slice between the bones.