Thaw the puff pastry sheets overnight in the refrigerator. Let them sit at room temperature for 15-20 minutes, ensuring they’re pliable but not sticky. Preheat your oven to 400°F (200°C) and prepare a baking sheet lined with parchment paper.

Cut the puff pastry into squares and place them on the prepared baking sheet. Brush each square with melted butter and sprinkle with a pinch of salt. Bake for 12-14 minutes until golden brown.

While the pastry is baking, chop the cooked chicken into small pieces. In a mixing bowl, combine the chicken with celery, grapes, and onion. Stir in the mayonnaise until well coated.

Allow the pastry squares to cool slightly. Cut each puff pastry in half horizontally for filling. Spoon the prepared chicken salad onto the bottom half and gently press the top half back on.

Arrange the assembled puffs on a platter. Best enjoyed warm for optimal texture and flavor.