In a small saucepan, place chicken breast and cover with water. Heat until boiling, then reduce it to medium and cover the pot. Cook chicken until it reaches an internal temperature of 165 degrees, about 12 to 15 minutes, depending on thickness.
Shred chicken and coat with Buffalo sauce. Set aside.
Lightly grease mini muffin pan(s) with cooking spray or butter. Preheat oven to 375 degrees.
Cut puff pastry into 12 squares per sheet. Tuck one square into each cup of the muffin tin.
Cut brie into 24 pieces. Place one piece atop each puff pastry. Top with Buffalo chicken (enough to mostly cover it without overflowing). Bake for 15 to 18 minutes, or until puff pastry edges are golden and no longer translucent.
Garnish with a drizzle of Ranch dressing and scallions.
