1. Combine ground sirloin, chopped cilantro, salt, and pepper in a bowl; mix just until combined. Divide sirloin mixture into 4 equal portions, gently shaping each into a 1/2-inch-thick patty (about 3 1/2 inches in diameter).
2. Heat a grill pan over medium-high heat. Coat pan with cooking spray. Add patties to pan; grill 4 minutes. Carefully turn patties; grill 4 minutes or until desired degree of doneness. Remove patties from pan. Add buns, cut sides down, to pan; grill 1 minute or until toasted.
3. Combine mayonnaise and Sriracha in a small bowl, stirring with a whisk. Spread 1 tablespoon mayonnaise mixture on bottom half of each bun; top each with 1 patty. Divide cilantro sprigs evenly among servings; top with top halves of buns.