Trim off the excess fat from the chicken thighs with a pair of kitchen scissors and cut them into chunks. Marinate the chicken with 1/2 tablespoon of chili powder and lemon juice. Set aside.
Turn on the Instant Pot and set it to Saute Mode. When the pot is fully heated, add the butter and melt it. Add the garlic and ginger, and stir a few times. Add the chicken and sauté until the surface turns white.
Add the remaining chili powder, tomato paste, ketchup, garam masala, salt, sugar, and water. Cover the pot, select Manual, and set it to High Pressure for 10 minutes.
When it beeps, turn to Quick Release. Once the valve drops, carefully remove the cover, add the heavy whipping cream and chopped cilantro, and stir to mix well.
Drizzle additional heavy cream on top of the Butter Chicken before serving, if desired.