Process ⅓ cup water, yogurt, garlic, ginger, paprika, 2 teaspoons salt, and cayenne in blender until smooth, about 2 minutes, scraping down sides of blender jar as needed. Transfer yogurt marinade to bowl.

Process tomatoes in now-empty blender until coarsely pureed, about 5 seconds. Transfer to second bowl.

Add chicken to bowl with yogurt marinade and toss to coat, rubbing marinade into chicken. Cover and refrigerate for at least 1 hour or up to 24 hours (if marinating longer than 1 hour, cover and refrigerate tomatoes as well).

Heat cumin seeds in 8-inch skillet over medium heat, stirring frequently, until fragrant, about 3 minutes. Transfer to spice grinder or mortar and pestle and grind to powder; set aside.

Heat oil in large Dutch oven over medium-high heat until just smoking. Add onion and remaining 1 teaspoon salt and cook until onion is softened, 5 to 7 minutes. Add tomatoes; increase heat to high; and cook until mixture darkens slightly and begins to stick to bottom of pot, 5 to 7 minutes.

Add chicken, along with any marinade left in bowl, and remaining 3 cups water and bring to boil. Cover; reduce heat to medium-low; and simmer, stirring occasionally, until chicken registers at least 175 degrees, 25 to 30 minutes.

Using tongs, transfer chicken to large plate. Increase heat to high and cook, uncovered and stirring occasionally, until sauce is thickened and reduced to about 2¾ cups, 12 to 15 minutes. Stir cumin into sauce. Add chicken back to pot and stir to coat with sauce. Serve, sprinkled with cilantro and passing extra yogurt separately.