Preheat oven to 400º F. Wash sweet potatoes and prick all over with the tines of a fork. Place on oven rack. Season chicken breasts generously on both sides with salt. Place in a baking dish and place in oven.
After 20-25 minutes, or when thermometer inserted in the thickest part of chicken breasts reaches 161º F, remove chicken from oven. Continue to bake sweet potatoes another 20-30 minutes or until soft.
While sweet potatoes bake, let chicken cool slightly. Shred chicken, either by hand with two forks or with a stand or hand mixer. In a small bowl, stir together all sauce ingredients until smooth. Pour over chicken and add white and light green parts of sliced green onions. Stir until well combined.
When sweet potatoes are soft, remove from oven and slice down the middle carefully. Push ends in, using a towel or potholder to protect your hands, to open the sweet potato a little. Stuff with buffalo chicken mixture and top with more ranch dressing and garnish with dark green parts of sliced green onions. Sprinkle with blue cheese crumbles if desired (and not on Whole30 or dairy free) and serve with celery sticks (optional).