In a medium bowl, combine chicken pieces with yogurt, 1 teaspoon garam masala, and 1/2 teaspoon salt. Mix well and let marinate for at least 15 minutes at room temperature.

Heat 2 tablespoons butter in a large skillet over medium-high heat. Add marinated chicken and cook for 6-8 minutes, stirring occasionally, until chicken is golden and nearly cooked through. Transfer chicken to a plate and set aside.

In the same skillet, add remaining 1 tablespoon butter. Add diced onion and cook for 4-5 minutes until softened and lightly golden.

Add minced garlic and grated ginger to the skillet and cook for 1 minute until fragrant.

Stir in tomato paste and remaining 1 teaspoon garam masala. Cook for 1-2 minutes until the tomato paste darkens slightly.

Pour in crushed tomatoes and bring to a simmer. Cook for 8-10 minutes, stirring occasionally, until the sauce thickens slightly.

Remove skillet from heat and slowly stir in heavy cream and brown sugar. Season with remaining 1/2 teaspoon salt.

Return the cooked chicken to the skillet and gently stir to combine. Return to low heat and simmer for 3-4 minutes until chicken is heated through and sauce reaches desired consistency.

Taste and adjust seasoning as needed. Garnish with fresh cilantro and serve immediately over basmati rice.