Remove the 2 racks costillas baby back aproximadamente 2–2.5 lbs cada una from the fridge 30 minutes before cooking to take the chill off. Preheat your Oven to 275°F. Line a large Baking Sheet (or two) with heavy-duty Aluminum Foil.

Flip the 2 racks costillas baby back aproximadamente 2–2.5 lbs cada una meat-side down. Locate the thin, papery membrane on the back of the rack. Use a Knife to loosen a corner, then grab it with a paper towel and pull it off in one firm motion. Removing this membrane is key to tender, flavorful ribs.

In a Mixing Bowl, combine all the spice rub ingredients: 3 tbsp azúcar moreno compactado, 2 tbsp paprika ahumada, 1 tbsp sal kosher, 1 tbsp sal kosher, 1 tbsp polvo de ajo, 1 tbsp polvo de cebolla, 1 tsp polvo de mostaza seca, and 1 tsp pimienta de cayena (if using). Stir until evenly mixed.

Brush a thin, even layer of 2 tbsp mostaza amarilla over both sides of each rack. The mustard acts as a binder and helps the rub adhere — you won't taste it after cooking.

Generously coat both sides of each rack with the spice rub, pressing it firmly into the meat. Use all of the rub — don't be shy. Place the racks meat-side up on the prepared Baking Sheet(s).

Tightly wrap each rack individually in two layers of Aluminum Foil, sealing all edges so no steam can escape. Place the foil packets back on the Baking Sheet, seam-side up.

Bake in the preheated 275°F Oven for 2.5 to 3 hours, until the meat is very tender and has pulled back about ½ inch from the ends of the bones. The low-and-slow method breaks down the connective tissue without drying out the meat.

Carefully open the Aluminum Foil packets (watch out for hot steam!) and check for doneness. The 2 racks costillas baby back aproximadamente 2–2.5 lbs cada una should be very tender when poked with a fork and the meat should have noticeably pulled back from the bone tips. If they need more time, reseal and bake another 20–30 minutes.

Preheat your Grill to medium-high heat (about 400°F). If using a Broiler instead, position the rack 6 inches from the broiler element and preheat on high.

Remove the 2 racks costillas baby back aproximadamente 2–2.5 lbs cada una from the Aluminum Foil and brush a generous layer of 1 ½ cups salsa Grill tu marca favorita o casera over the meat side of each rack. Reserve some sauce for serving.

Place the racks meat-side down on the hot Grill. Brush 1 ½ cups salsa BBQ tu marca favorita o casera on the bone side. Grill for 3–4 minutes until the sauce caramelizes and develops char marks. Flip, brush the meat side again with sauce, and grill another 3–4 minutes. Watch carefully — the sugar in the sauce can burn quickly.

Transfer the racks to a Cutting Board and let them rest for 5 minutes. Slice between the bones into individual ribs. Serve with extra 1 ½ cups salsa BBQ tu marca favorita o casera on the side.