Creating Rich Butter Chicken Biryani is easy if you follow these steps:

1. Marinate the Chicken: In a bowl, combine the chicken pieces with yogurt, ginger-garlic paste, turmeric powder, red chili powder, coriander powder, cumin powder, and salt. Let it marinate for at least 30 minutes.

2. Cook the Rice: Rinse the basmati rice under cold water until the water runs clear. Soak it in water for 30 minutes. In a pot, bring 4 cups of water to a boil. Add the soaked rice and cook until it is 70% done. Drain and set aside.

3. Fry Onions: In a large, heavy-bottomed pot, heat the vegetable oil and 2 tablespoons of butter. Add the sliced onions and fry until they turn golden brown. Remove half of the fried onions and set aside for garnishing.

4. Cook the Chicken: In the same pot, add the marinated chicken and cook on medium heat until the chicken is cooked through and the oil begins to separate from the mixture. Add chopped tomatoes and cook until soft.

5. Layer the Biryani: Once the chicken is cooked, reduce the heat to low. Layer the partially cooked rice over the chicken. Sprinkle garam masala, slit green chilies, and the reserved fried onions on top. Drizzle the remaining butter over the rice.

6. Dum Cooking: Cover the pot with a tight-fitting lid. Cook on low heat for about 20-25 minutes to allow the flavors to meld. You can place a heavy pan on top of the lid to trap the steam.

7. Garnish and Serve: Once done, fluff the biryani gently with a fork. Garnish with fresh cilantro and mint leaves before serving.