Mix salt and sugar together in a small bowl. Sprinkle all over ribs. Coat with red chilli flakes, five-spice powder and black pepper. Slide ribs into a large vacuum-seal bag; pour in liquid smoke. Seal.
Immerse bag in a heat-proof container of water with a sous vide cooker. Set temperature to 60 degrees C; cook for 8 hours.
Preheat oven to 230 degrees C. Set oven rack about 15cm from the heat source and turn on the grill to medium-high. Place a wire rack on top of a baking tray.
Pull ribs out of the bag; spread onto the prepared baking tray and brush with barbecue sauce.
Bake in the preheated oven until bubbly, 4 to 5 minutes. Flip and continue baking until other side bubbles, 2 to 3 minutes more.