Indian Butter Chicken (Murgh Makhani)

1 onion , diced

4 large garlic cloves , minced

2 tablespoons butter

1 (13 1/2 ounce) can coconut milk (not the light kind)

1 (5 1/2 ounce) can tomato paste

1 cup plain yogurt

2 tablespoons tandoori masala paste

1 tablespoon mild curry paste (such as Pataks mild curry paste)

2 teaspoons curry powder

1 1/2 teaspoons garam masala

15 green cardamom pods (tied up in cheesecloth for easy removal)

1/2 teaspoon cayenne pepper (optional- for spicier Murgh Makhani)

salt