Combine all marinade ingredients and allow to sit in fridge overnight, or at least for a few hours.
Heat butter in large skillet over medium heat. Once melted, add diced yellow onion and serrano peppers. Cook until the onions start to soften, about 3-4 minutes.
In a small bowl, mix together the garam masala, turmeric, smoked paprika, cumin, coriander, and salt.
Add the spices from step 3, minced garlic, and fresh ginger to the skillet. Stir together, coating the onion and peppers in the spices. Once the spices are aromatic, 1-2 minutes, remove from heat. Be careful not to cook the spices for too long--you don't want to burn them!
Add chicken marinade and onion/pepper mixture to the slow cooker. Add the brown sugar and crushed tomatoes, then mix well.
Put lid on slow cooker and cook on low for 5-6 hours, or on high for 3 hours.
Stir in the heavy cream, and cook for another 15 minutes. (Add the extra ½ cup of heavy cream if you feel that it needs it)
When done, garnish with cilantro. Enjoy!