Heat butter in skillet over medium heat. Add chicken and sear on each side for 2-3 minutes until lightly browned. Add to slow cooker with remaining ingredients except for fresh cilantro and cheese. Stir.
Cover and cook on low for 7-8 hours. Serve with fresh cilantro and cheese.
If using a dried ancho chili instead of a fresh poblano, sear it in a dry skillet for about a minute on each side and then add it to boiling water and let sit for 30 minutes until it is soft enough to chop.