Refer to note #1 for instructions on removing the membrane from baby back ribs. This step is crucial to avoid tough, chewy ribs.
In a bowl, combine all the spices. Stir well with a spoon, pressing out any large clumps.
Tear two sheets of aluminum foil, each long enough to extend beyond the sides of each rack of ribs. Place one rack curved side down (with the side where the membrane was removed facing down) on each piece of foil. Using a brush, spread 1 tablespoon of yellow mustard over each rack. Sprinkle one-quarter of the dry rub over the ribs, then spread it evenly with your fingers. Flip the racks over and repeat the mustard and dry rub application on the other side.
Once both racks are well coated, carefully wrap the foil around them. For extra sealing, add a second sheet of foil around the center to ensure the ribs are completely enclosed (see note #3). Position the wrapped ribs curved side down, so the membrane-removed side faces down.
Place the ribs in the refrigerator to marinate for 2 hours.
Preheat the oven to 275°F. Remove the ribs from the refrigerator and place them on a baking sheet curved side down (membrane-removed side facing down). Bake for 2 1/2 hours. If any liquid starts to leak onto the baking pan, carefully add more foil to wrap them securely.
Preheat the grill to 350°F. After baking, remove the ribs from the oven and carefully open the foil. Use a meat thermometer to check the temperature between the bones. While a safe internal temperature is 145°F, ribs will become fall-off-the-bone tender at 190°F to 200°F.
Brush your preferred BBQ sauce over the top of the ribs. Because the ribs will be very tender, handle them carefully to prevent falling apart—use two sets of tongs, one on each side, when transferring and flipping. Place the ribs sauce-side down on the grill. As that side chars, brush more BBQ sauce on the exposed side. Grill for about 5 minutes on each side, until you see charred grill marks. Remove from the grill.
To serve, position the ribs curved side up so the bones are visible. Slice between the bones and serve with additional BBQ sauce.