In a medium bowl, combine the chicken cubes with all the marinade ingredients: yogurt, lemon juice, garam masala, turmeric, cumin, chili powder, ginger, and garlic. Mix thoroughly to ensure every piece of chicken is coated. Cover and let it marinate for at least 30 minutes, or up to 4 hours in the refrigerator for deeper flavor.

In a large skillet or Dutch oven, heat one tablespoon of butter or ghee over medium-high heat. Remove the chicken from the marinade, allowing any excess to drip off. Cook the chicken in batches until golden brown and cooked through, about 5-7 minutes. Set the cooked chicken aside on a plate.

In the same skillet, melt the remaining tablespoon of butter. Add the chopped onion and cook until softened, about 5 minutes. Stir in the ginger and garlic and cook for another minute until fragrant. Pour in the crushed tomatoes, garam masala, coriander, and sugar. Season with salt and pepper. Bring the sauce to a simmer and cook for 10 minutes, stirring occasionally, until it thickens slightly.

Reduce the heat to low and stir in the heavy cream. Return the cooked chicken to the skillet, coating it completely in the sauce. Let it simmer gently for 5 more minutes, allowing the flavors to meld together. Be careful not to let the sauce boil after adding the cream.