Spatchcocked Buffalo Chicken

Name: Spatchcocked Buffalo Chicken
Ingredients: 1 3½-to-4-lb. whole chicken, patted dry; ¼ cup hot sauce, such as Tabasco; 1 tablespoon smoked paprika; 3 ½ cups whole buttermilk, divided; 4 teaspoons kosher salt, divided; 1 ½ teaspoons plus a pinch freshly ground black pepper, divided; 1 tablespoon olive oil; 2 ounces blue cheese, crumbled (about ½ cup); 2 tablespoons finely chopped fresh flat-leaf parsley; 1 tablespoon finely chopped shallot; 1 tablespoon fresh lemon juice (from 1 lemon); 2 teaspoons Dijon mustard; 1 pound fresh yellow wax beans, blanched; 2 stalks celery, thinly sliced (¾ cup); 1 cup celery leaves