In a small bowl, mix together the brown sugar, 2 teaspoons salt (Diamond Crystal kosher, but adjust based on your salt’s saltiness) black pepper, garlic and chili flakes. Place the rack of ribs on a cutting board or sheet pan then thoroughly rub the spice mixture all over the rack.
Pour the chicken broth and apple cider vinegar into the bottom of the Instant Pot®. With the wire rack on top. Then place the ribs inside, bending the rack to the curvature of the pot. Secure the lid, make sure the steam nozzle is closed and cook on high pressure for 25 minutes. When the time is up, let the pressure release naturally for about 10 minutes then use the quick release option for the remaining pressure.
While the ribs are cooking, add all of the barbecue sauce ingredients to a small pot, stir and bring to a simmer. Cook, stirring often, for about 5 minutes then turn off the heat.
Line a sheet pan with parchment or foil and carefully transfer the cooked ribs onto the sheet pan. Brush rack all over with the barbecue sauce and transfer the remaining sauce to a small bowl.
Turn your broiler on and place the sheet pan with ribs under it. Adjust the pan as needed to evenly brown the sauce and ribs.
When ready to serve, slice the ribs individually or in sections and plate with the barbecue sauce and plenty of napkins nearby.