Begin by setting a 6-quart Instant Pot® to high sauté mode. Melt 4 tablespoons of the butter in the pot. Incorporate the ginger, fenugreek leaves, fenugreek seeds, garam masala, garlic, cardamom, onion, serrano chile, the cinnamon stick, and 1 tablespoon of salt. Cook this mixture, stirring occasionally, until the onions become tender and start to brown, which should take approximately 8 to 10 minutes. Add the tomato paste, stirring frequently while scraping the bottom of the pot to prevent burning, until the paste develops a rich brick-red hue.

Next, deglaze the pot with 3 cups of water, ensuring that the ingredients are well combined. Layer the peeled tomatoes on top, followed by the chicken thighs.

Following the manufacturer’s instructions, lock the lid in place and prepare to cook. Set the Instant Pot to pressure cook on high for 10 minutes.

Once the pressure-cooking cycle is complete, perform a quick release according to the manufacturer's guidance, allowing the quick-release cycle to finish. Carefully unlock and remove the lid, being cautious of any residual steam.

Discard the cinnamon stick and transfer the chicken to a plate, covering it loosely with foil until ready to serve. Return the pot to high sauté mode and continue to simmer the sauce, stirring occasionally until it has reduced by half, which should take about 25 to 30 minutes. Afterward, turn off the pot and puree the sauce using an immersion blender. Alternatively, if using a traditional blender, let the sauce cool for a few minutes before blending until smooth.

Stir in the cream and the remaining 4 tablespoons of butter, blending until fully incorporated and the mixture is smooth and creamy with no visible streaks of cream. Cut the chicken into 1-inch pieces and fold it back into the sauce. Allow it to sit for approximately 5 minutes to heat through. Serve the butter chicken garnished with chopped cilantro over a bed of steamed basmati rice, accompanied by naan.