Preheat the oven to 275°F / 135°C and line a large sheet pan with foil.
Mix together the sugar, onion powder, garlic power, smoked paprika, salt and pepper.
Lay the racks of ribs bones up on the baking sheet and brush generously with the mustard. Sprinkle half of the sugar and spice mixture over the ribs and press into the ribs.
Flip the ribs over to have the meaty side facing up and repeat with the mustard and sugar and spice mixture.
Cover with foil and place into the oven for two and a half hours.
Remove from the oven and remove the foil. Brush generously with the barbecue sauce then return to the oven uncovered for a further 30 minutes.
Allow to rest for five minutes once removed from the oven then slice into individual ribs.
Serve with additional barbecue sauce on the side and buttered corn on the cobs.