Make the hoisin BBQ sauce by combining the hoisin sauce, chili sauce, dry sherry, honey, soy sauce, rice vinegar, sesame oil, minced garlic, and ground ginger in a medium bowl. Whisk until smooth.
Cut each full rack of ribs in half, creating 6 or 7 rib sections. Line a large, rimmed baking sheet with heavy duty aluminum foil.
Place the rib sections on the prepared baking sheet. Reserve 1 cup of the sauce for glazing later. Pour the remaining sauce over the ribs, using your hands or a brush to coat both sides evenly. Arrange them meaty side up in a single layer.
Cover the baking sheet tightly with another piece of heavy duty aluminum foil. Refrigerate to marinate for at least 4 hours, or ideally overnight.
Preheat your oven to 300°F (150°C) and set a rack in the center position. Place the foil covered ribs in the oven and bake undisturbed for 2 hours.
Carefully remove the pan from the oven. Be cautious of the hot steam when you peel back the foil. Increase the oven temperature to 350°F (175°C).
Generously brush the reserved 1 cup of sauce all over the top of the ribs. Return the pan to the oven, uncovered, and bake for another 25-30 minutes, until the sauce is thick, sticky, and caramelized.
Let the ribs cool for 10-15 minutes before slicing between the bones to serve. Garnish with sliced scallions and sesame seeds, if desired.