Prepare and Marinate Oxtails: Start by rinsing the oxtail pieces under cold water mixed with a splash of vinegar, then pat them dry with paper towels. In a large bowl, combine the brown sugar, soy sauce, Worcestershire sauce, salt, garlic powder, black pepper, allspice, and browning sauce. Coat the oxtails thoroughly with this spice mixture and let them marinate for at least 15 minutes to absorb the flavors.
Brown the Oxtails: Set your pressure cooker to the sauté function and heat the vegetable oil until shimmering. Carefully place the oxtail pieces in the pot, arranging them flat side down without crowding, and brown each side until they develop a rich, caramelized crust (about 3-4 minutes per side). Once browned, transfer the oxtails to a bowl and set aside.
Sauté Aromatics and Vegetables: In the same pot, pour in about 2 tablespoons of beef broth to deglaze the bottom, scraping up all the browned bits with a wooden spatula for extra flavor. Add the yellow onion, green onions, garlic, carrots, and chopped scotch bonnet pepper. Stir frequently and cook for about 5 minutes, or until the onions soften and become translucent.
Combine Ingredients and Pressure Cook: Return the browned oxtails to the pot along with any accumulated marinade juices. Pour in the remaining beef broth and ketchup, then sprinkle in the dried thyme. Stir gently to combine, then seal the pressure cooker lid and set it to cook at high pressure for 45 minutes. After cooking, allow the pressure to release naturally before opening the lid.
Thicken Sauce and Add Butter Beans: Carefully remove the oxtails and vegetables from the pot and place them on a serving dish. Turn the pressure cooker back to sauté mode and bring the liquid to a simmer. In a small bowl, mix the cornstarch with water to create a smooth slurry, then slowly stir this into the simmering sauce. Add the drained butter beans and cook for about 5 minutes, until the sauce thickens and the beans are heated through.
Recombine and Serve: Return the oxtails and vegetables to the pot to warm through with the thickened sauce. Serve hot and enjoy the rich, comforting flavors of this traditional Jamaican stew.