Toss the chicken with 1 1/2 teaspoons salt and a few grinds of pepper in a large bowl. Set aside at room temperature. Stir the garlic, ginger, garam masala, paprika, cumin, turmeric and cayenne with 2 tablespoons water in a small bowl.
Set an Instant Pot to sauté on high. When the pot registers hot, add 2 tablespoons butter and let melt. Add the spice mixture and cook, stirring constantly, until thickened and sticking to the bottom of the pot, 2 to 3 minutes. Add the tomato puree, sugar and 1/2 teaspoon salt, scraping up the bottom of the pot. Turn off the pot.
Add the chicken to the pot; stir to coat. Put on and lock the lid; make sure the steam valve is in the sealing position. Set the pot to pressure-cook on high for 5 minutes. When the time is up, let the pressure release naturally for 10 minutes, then carefully turn the steam valve to the venting position to manually release the remaining pressure. Turn off the pot and remove the lid.
Stir the remaining 4 tablespoons butter and the heavy cream into the pot; season with salt and pepper. Divide the chicken and sauce among bowls. Top with cilantro and serve with naan or rice.