Season and sear the chicken: Combine garlic powder, paprika, and salt in a small bowl. Sprinkle spice mixture over top (smooth) side of chicken thighs. Heat a 14-inch skillet over medium-high. Add oil to pan and swirl to coat. Add chicken to pan, seasoned side down. Cook until seared, about 3 minutes.
Prepare sauce and cook: Place chicken, seared side up, in a 6-quart slow cooker. Whisk together honey, soy sauce, vinegar, ginger, and garlic in a medium bowl. Pour honey mixture over chicken. Sprinkle butter pieces over the top. Cover and cook until chicken is tender, 5 to 6 hours on Low or 3 to 4 hours on High.
Finish sauce and serve: Remove chicken from slow cooker. Whisk together cornstarch and 3 tablespoons water in a small bowl; stir into sauce in cooker. Cook until slightly thickened, 3 to 5 minutes. Serve chicken over rice; top with sauce and green onions.
