Mix baking soda and salt together. Score the skin of the pork belly. 1/4 inch deep every 1/2 inch. Rub the skin with salt mixture. Put belly on a sheet pan with a rack and place uncovered in the refrigerator overnight. Up to 24 hours.
Preheat oven to 400 degrees. Roast poblanos for 15-20 minutes. Place in a bowl and cover with plastic wrap. When the peppers have cooled, peel off the skin. Deseed and mince the peppers.
Preheat oven to 275. Trim the fat from the pork butt and cut into 1 inch cubes. Score the meat side of the pork belly 1/2 inch deep in a 1 inch cross pattern. Season the meat of the belly and pork butt with poblanos, garlic, salt, pepper, smoked paprika, and Aleppo pepper.
Fill the belly with the butt. Form the belly into a cylinder around the butt. Place seam side down and tie with butcher's twine every 1 1/2 inches.
Place pork on a sheet pan with a rack and roast for 2-2 1/2 hours.. Pork should be 140 degrees. Turn the oven to 500 degrees and crisp the skin for 15-20 minutes. Remove from pan and rest 10-15 minutes before slicing and serving.
