1. In a medium-size microwave-safe bowl, stir marmalade with soy sauce. Microwave, uncovered, on high until softened, 1 minute. Or melt in a saucepan, stirring often. Stir in barbecue sauce. Place chicken in bowl of slow cooker. Pour in sauce. Stir to coat. Cover and cook on high setting 4 hours or low 6 hours. To bake, preheat oven to 350F (180C). Place chicken in a large, ovenproof casserole dish that will hold at least 16 cups (4 L). Pour sauce over chicken. Stir to evenly coat. Set on a baking sheet. Bake, covered, in centre of preheated oven until chicken feels springy when pressed and sauce is bubbly, about 1-3/4hours.
2. To thicken sauce, remove cooked chicken to a large bowl. In a small bowl, stir cornstarch with water until dissolved. Carefully ladle liquid from slow cooker or casserole into a medium-size saucepan. Set over high heat. Bring to a boil. Stir in cornstarch mixture. Reduce heat to medium. Stir until sauce thickens, 2 minutes. Pour over chicken.