Preheat the oven to 275°F (135°C).
In a large mixing bowl, combine the brown sugar, smoked paprika, garlic powder, onion powder, chili powder, salt, black pepper, and cayenne pepper if using. Add the pork belly cubes and toss until every piece is evenly coated with the seasoning mixture.
¼ cup (55 g) packed light brown sugar, 2 tbsps (14 g) smoked paprika, 1 tbsp (9 g) garlic powder, 1 tbsp (8 g) onion powder, 1 tbsp (8 g) chile powder, 2 tsps (12 g) table salt, 1 tsp (2 g) black pepper, ½ tsp (0.5 g) cayenne pepper, 4 lbs (1.8kg) skinless pork belly
Arrange the pork belly in a single layer on a large rimmed baking sheet. Cover tightly with aluminum foil and bake for 2½ to 3 hours, or until the pork is fork tender and reaches an internal temperature of 190 to 195°F (88 to 90°C). A fork should slide into the cubes with very little resistance.
While the pork bakes, pour the peach tea into a medium saucepan and bring it to a boil over medium-high heat. Reduce the heat to medium and simmer for 15 to 20 minutes, or until the tea has reduced by about half and the peach flavor becomes more concentrated.
2 cups (480 ml) bottled sweeteened peach tea
Whisk in the peach preserves, apple cider vinegar, brown sugar, Worcestershire sauce, and salt. Continue simmering for 4 to 5 minutes, whisking occasionally, until the preserves have completely melted into the glaze and no visible pieces remain.
¾ cup (255 g) peach preserves, ¼ cup (60 ml) apple cider vinegar, 2 tbsps (28 g) packed light brown sugar, 1 tbsp (15 ml) Worcestershire sauce, 1 tsp (6 g) table salt
In a small bowl, stir together the cornstarch and cold water until smooth. Slowly whisk the slurry into the simmering glaze and cook for 1 to 2 minutes, whisking frequently, until the glaze becomes shiny and lightly coats the back of a spoon instead of running right off. Remove the saucepan from the heat.
2 tsps (5 g) cornstarch, 2 tsps (10 ml) cold water
Carefully remove the foil from the baking sheet, watching for escaping steam. Transfer the pork belly cubes to a large bowl with tongs, then carefully pour the rendered fat from the baking sheet into a heat-safe container.
Increase the oven temperature to 375°F (190°C).
Reserve about ½ cup (120 ml) of the glaze. Pour the remaining glaze over the pork belly and gently toss until every cube is evenly coated.
Return the glazed pork belly to the baking sheet in a single layer. Bake uncovered for 15 to 20 minutes, or until the glaze is bubbling around the edges, beginning to caramelize, and the pork reaches an internal temperature of 200 to 205°F (93 to 96°C).
Brush or gently toss the pork belly with the reserved glaze. Let the burnt ends rest for 5 minutes so the glaze has time to become sticky before serving.