Optional: Chopped scallions and toasted sesame seeds for garnish
In a bowl, whisk together 1 TB cornstarch, soy sauce, granulated sugar, brown sugar, mirin, garlic, ginger, and black pepper. Whisk until smooth.
Place the chicken thighs in a single layer in the slow cooker. Pour enough sauce to almost cover the top of chicken. (Save any leftover for boiling after the chicken is cooked or keep it in fridge for future use.)
Cook on high for 3 hours, on low for 5 hours, or just until chicken is cooked through. To keep meat tender, don't overcook.
Remove the chicken with slotted spoon, and gently shred or cut into pieces. (Don't shred too finely, as it will break up more in the sauce.)
Transfer the sauce from the slow cooker into a saucepan. Dissolve 2 TB cornstarch in 2 TB water. Add the mixture to the saucepan, stirring well. Bring to a low boil, stirring continuously, until sauce is thickened.
Pour sauce over chicken and rice. Garnish with green onions and sesame seeds, if desired.