In a large skillet over medium-high heat, toast the pepitas (pumpkin seeds), stirring constantly until fragrant (2-3 minutes). Remove from heat.
In a blender or food processor, combine the onion, garlic, tomatoes, raisins, cumin, ancho chile powder or chilies, chili powder, oregano, chipotle chilies, adobo sauce, Mexican chocolate, oil and toasted pepitas. Pulse to form a thick sauce will all ingredients pureed.
Rinse the chicken thighs and pat dry with paper towel. Pour a large amount of kosher salt and ground pepper into a bowl or dinner plate and coat both sides of each chicken thigh. Place chicken thighs in the slow cooker.
Cover the chicken with the sauce and move the chicken to allow the sauce to reach the lower pieces of chicken without removing the salt and pepper coating.
Cook on high for 4-6 hours until chicken is fully cooked, stirring a few times during the cooking to ensure the sauce is coating each chicken thigh.
Serve over rice or sliced in tacos. May be refrigerated and used the following day as well.