Thaw the puff pastry in the refrigerator overnight. On the day of assembly, allow it to sit at room temperature for about 15-20 minutes.

Preheat your oven to 400°F (200°C). Cut the pastry into 36 equal squares, arrange them on a lined baking sheet, brush with melted butter, and sprinkle with salt. Bake for 12-14 minutes until golden.

While baking, grill the chicken breasts over medium heat, about 6-7 minutes per side, until fully cooked. Let cool and chop into small pieces.

In a large mixing bowl, combine chopped chicken, celery, grapes, and onion. Add mayonnaise and mix gently to combine.

Fill each pastry shell with a tablespoon of chicken salad, secure the top, and use toothpicks if needed.