In a blender, puree the cashews and ¾ cup water until smooth, 1 to 2 minutes.
Transfer to a small bowl; set aside if pressure cooking or cover and refrigerate if slow cooking.
In the now-empty blender, puree the tomatoes with their juices until smooth, about 1 minute.
On a 6-quart Instant Pot, select Normal Sauté.
Add 2 tablespoons of the butter and melt.
Add the onion and 1 teaspoon salt; cook, stirring occasionally, until softened, 3 to 5 minutes.
Add the garlic, ginger, garam masala, cumin and chili powder; cook, stirring, until fragrant, about 30 seconds.
Stir in the chicken and tomato puree, then distribute in an even layer.
Stir the cashew puree and remaining 1 tablespoon butter into the chicken mixture, scraping the bottom of the pot.
Select Less/Low Sauté and cook, stirring often, until the sauce has thickened and coats the chicken, 4 to 5 minutes.
Using potholders, carefully remove the insert from the housing. Cool for about 5 minutes.
Stir in half of the cilantro, then taste and season with salt and pepper.
Serve sprinkled with the remaining cilantro.
