Set sous vide machine to 74C/165F.
Remove the membrane from the backside of the rack of ribs. Do this by sliding a table knife under the membrane anywhere along the rack. Lift and loosen it with the knife until you can grab it with a paper towel. Pull it off the ribs; it should peel away in one large sheet, but if it breaks, use the knife to restart at another section.
Lightly season the rack of ribs with salt. Put the ribs in a freezer bag, and remove the air through a vacuum sealer or the displacement method. Note: the ribs may have to be cut in half to fit in to bag. Place rack of ribs in the bath, and cook for 12 hours.
Take ribs out of the bath and pat dry with paper towel. Combine chili powder, paprika, cumin, brown sugar, pepper, salt and cayenne pepper in a small bowl, then season generously on both sides.
Turn grill on high. Place ribs on grill, meat side down, for 5 minutes. Flip the ribs over and coat the top with your favorite BBQ sauce. After 5 more minutes, take the ribs off the grill and coat with BBQ sauce again. Let rest for 5 minutes, and enjoy the most tender, flavorful ribs of your life!
Alternative instruction - If you do not have a grill, you can also finish the ribs in the oven. Set oven to 350F, and bake for 10 minutes. Take ribs out and coat the rack with your favorite BBQ sauce. Place ribs back in oven for 10 more minutes. Take the ribs out of the oven and coat with BBQ sauce again. Let rest for 5 minutes, and enjoy!