Step 1

To marinate chicken: Combine garlic and ginger in a bowl. Press with a spoon to make a paste. Transfer 2 packed tablespoons to a large bowl and add yogurt, serrano, chickpea flour (if using), 1 tablespoon coriander, 1 teaspoon chili powder, garam masala, ground cinnamon, cumin, turmeric, 1/2 teaspoon salt and saffron; whisk until well combined. Add chicken and stir to coat. Cover and refrigerate at least 2 hours and up to 1 day. Refrigerate the remaining garlic-ginger paste.

Step 2

To prepare chicken: Position a rack in upper third of oven; preheat broiler to high. Line a baking sheet with foil.

Step 3

Spread the chicken on the prepared baking sheet. Broil until the chicken is cooked through and starting to brown, 10 to 12 minutes.

Step 4

To prepare sauce: Heat oil in a large saucepan over medium heat. Add cardamom, cloves and cinnamon sticks; cook, stirring, until fragrant, about 1 minute. Add onion and cook, stirring occasionally, until translucent, 3 to 5 minutes. Stir in serrano, fenugreek, coriander, garam masala, chili powder and the reserved garlic-ginger paste. Cook, stirring, for 1 minute. Stir in tomato sauce and salt; cook for 2 minutes more. Add water and cashew butter; increase heat to medium-high and cook, stirring often, until the sauce looks creamy, about 5 minutes. Add the chicken and any accumulated juice to the sauce. Cook, stirring, until heated through, 2 to 3 minutes. Serve with sliced serrano chile, if desired.