For the chicken: Combine salt, garam masala, and cayenne pepper in a small bowl. Dry chicken dry with paper towels and season with spice mixture. Place chicken on plate, cover and refrigerate for 30 minutes to 1 hour. Whisk yogurt, oil, and ginger together in medium bowl, cover, and refrigerate until needed.

For the sauce: Process tomatoes and their juice in food processor until smooth, about 15 seconds. Heat oil in large saucepan over medium heat until shimmering. Add onion and cook until softened and lightly browned, 5 to 7 minutes. Stir in serrano, tomato paste, garam masala, garlic, and ginger and cook until fragrant, about 30 seconds. Stir in processed tomatoes, sugar, and ½ teaspoon salt and bring to simmer. Reduce heat to low, cover, and simmer, stirring occasionally, until flavors meld, about 15 minutes. Off heat, stir in yogurt, cover to keep warm.

Meanwhile, adjust oven rack 6 inches from broiler element and heat broiler. Set wire rack in aluminum foil-lined rimmed baking sheet. Using tongs, dip chicken into yogurt mixture (chicken should be coated with thick layer of yogurt) and arrange on prepared wire rack. Discard excess yogurt mixture.

Broil chicken until highly charred in spots and chicken registers 160 degrees, 10 to 18 minutes, flipping chicken halfway through cooking.

Transfer chicken to cutting board, let rest for 5 minutes, then cut into 1-inch pieces. Stir chicken pieces into warm sauce (do not simmer chicken in sauce). Stir in cilantro, season with salt to taste, and serve.