Add the vegetable oil to a heavy-bottomed saucepot set over high heat.
When the oil is barely smoking, add the marinated chicken and sear it on all sides until brown and crispy. (The chicken does not need to be cooked through.) Using a slotted spoon, remove the chicken and set it aside while you make the curry sauce.
In the same stockpot set over medium-high heat, add the butter. Once the butter has melted, add the onions, ginger and garlic. Cook them for 5 to 10 minutes, stirring occasionally, until the onions are translucent.
Add the tomato paste, chopped tomatoes and remaining spices and continue cooking for an additional 5 minutes. Allow the curry sauce to cool slightly then transfer it to a blender.
Purée the curry sauce until smooth then return it to the saucepot set over medium-low heat.
Stir in the heavy cream then bring the curry to a gentle simmer. Add the chicken to the sauce and simmer, covered, for 30 minutes until the chicken is cooked fully. Add the sugar and season to taste with salt and pepper. Serve the Chicken Tikka Masala over basmati or with naan.