In large saucepan over medium heat, heat the butter until frothy. Add the celery and onions and saute until the vegetables are tender, about 4 minutes. Season with the salt and pepper.
Whisk in the tomato paste until well combined, then add the broth and Buffalo sauce. Bring to a boil, then reduce the heat to a simmer.
Place the cream cheese in a blender or food processor and add about 1 cup of the hot broth. Blend until smooth, then add the cream cheese mixture to the saucepan.
Stir in the cream and bring back to just a simmer, then stir in the chopped chicken and heat briefly to warm through. Divide the soup among 6 bowls and garnish with chopped celery and feta cheese.