In a medium bowl, toss chicken with 1 tablespoon paprika, 1 1/2 teaspoons salt, and 1/4 teaspoon pepper. Heat 1 tablespoon butter in a large skillet over medium-high heat. Add chicken; cook, tossing occasionally, until opaque throughout, 4 to 5 minutes. Transfer to a plate.
Heat remaining tablespoon butter in same skillet over medium heat. Add onion; cook, stirring and scraping up browned bits from bottom of pan, until softened, 5 to 6 minutes. Add remaining tablespoon paprika, and cook, stirring, 30 seconds.
Add tomatoes and 3/4 cup water; cook until saucy, 4 to 5 minutes. Return chicken (and any accumulated juices) to skillet. Stir in sour cream, and cook just until heated through (do not boil). Season with salt and pepper; serve.