Heat oil in a large skillet over medium heat; add chicken pieces and cook until browned, about 8 minutes per side. Set chicken aside.

Place spinach into a large pot with 1/4 cup of water; bring to a boil, cover, and remove from heat. Allow spinach to steam for about 10 minutes; transfer into a blender and pulse until very finely chopped. Set spinach aside.

Place onions, garlic, and ginger into the skillet; fry over medium heat, stirring frequently, until lightly browned, about 10 minutes. Pour in tomatoes, ground coriander, salt, cayenne pepper, turmeric, cardamom, and cloves; stir in 1 tablespoon water, and cook, stirring frequently, for 10 minutes.

Pour in milk and stir to combine. Add browned chicken pieces; bring to a simmer, reduce heat, cover, and simmer until chicken is tender, about 20 minutes. Stir in spinach and garam masala; cook until spinach starts to stick to the skillet, about 15 minutes.

Stir in sour cream and bring to a simmer. Remove from heat and stir in butter.