Sprinkle salt and pepper onto the chicken thighs (start with 1 tsp salt and ½ tsp pepper, then add more to desired taste at the end). Set aside.
Using a food processor, puree garlic, onion, and ginger into a smooth paste
Mix cumin, garam masala, turmeric powder, smoked paprika, and cayenne pepper in a separate bowl
Press the [SAUTE] button. Add oil and butter into the inner pot
Once the butter is melted, add the pureed garlic/onion/ginger mixture and cook it for 1 minute, stirring occasionally.
Then, deglaze the pot by adding chicken broth. Scrape any bits stuck to the bottom of the pot to avoid the "burn" message.
Now without stirring, layer in the ingredients in the following order: chicken thighs (spread evenly in the pot), spice mixture (sprinkle evenly over the chicken thighs), and tomato paste.
Close the lid and turn the steam release handle to the “sealing” position. Press [PRESSURE COOK] (or [MANUAL] for older models) and set the instant pot to cook on HIGH pressure for 12 minutes.
Once it is done cooking, QUICK RELEASE the pressure by carefully turning the steam release handle to “venting”. Open the lid and press the [SAUTE] button. Add heavy cream, and additional salt and pepper to taste. Cook for approximately 2-3 minutes (stirring occasionally) or until the butter chicken reaches your desired consistency. If you wish to thicken up the gravy even further, you can use cornstarch as described in the notes below.