Heat oil, and saute onion. Add the garlic, ginger, cinnamon, bay leaves, coriander, turmeric, cayenne, and cumin. Cook, stirring, until the spices are fragrant and evenly coat the onion, about 1 minute.
Stir in the broth, tomato sauce, sugar, and 1 teaspoon salt and deglaze the pan. Bring the sauce to a boil.
Transfer the sauce to the slow cooker. Add the chicken and cook on low heat for 4 hours. The chicken should be tender.
Remove the chicken from the slow cooker. Shred the meat and remove any bones. Return the meat to the slow cooker.
About 15 minutes before the chicken is ready, combine the buttermilk and cashews in a blender or food processor. Blend until the nuts are finely pureed and thoroughly mixed with the buttermilk. Add to the chicken mixture, and stir to mix well.
Continue to cook on low heat until the sauce is heated through and thick, about 5 minutes. Remove and discard the cinnamon stick and bay leaves.