Combine ¾ cups cornstarch, the salt, and the pepper in a shallow dish. Dip the chicken pieces in the cornstarch mixture to coat, shake off excess, and set aside.
Heat the oil in a skillet over medium-high heat. Brown the chicken pieces. It should take less than 2 minutes per side. Remove the chicken and set aside.
In the slow cooker, combine the sugar, soy sauce, pineapple juice, vinegar, garlic, ginger, and cayenne. Stir to mix well.
Add the chicken to the crock pot and cook on low for 4 hours. Check 20 minutes before serving and make sure your sauce is thick enough. If not, take the remaining 2 tablespoons of cornstarch and mix it with 2 tablespoons cold water until a slurry forms. Then, gently mix a bit into the chicken and sauce, being careful not to scrape the cornstarch coating off of the chicken. If it's very thin, you'll need more, if it's not too thin, use is sparingly. It thickens significantly as it cooks, so you may not need the whole ¼ cup. Cook for another 20 minutes, and then serve over broccoli and rice. Top with chopped green onion.
