Dissolve bouillon in hot water; set aside.

Melt butter in a large saucepan over medium heat. Add onion, garlic, and pepper flakes; cook and stir until onion soft. Add squash, 2 tablespoons cilantro, salt, and black pepper; stir continually for 5 minutes.

Stir in bouillon mixture and remaining 1 tablespoon cilantro; cover. Simmer about 20 minutes.

Fill blender halfway with soup. Cover and hold lid down with a potholder; pulse a few times before leaving on to purée until smooth. Pour into a pot. Repeat with remaining soup.

Pour into bowls and garnish with a sprig of cilantro to serve.