Thaw your puff pastry by placing it in the fridge overnight. 15-20 minutes before using, let it sit at room temperature. Preheat your oven to 400°F (200°C), cut the pastry into 12 equal squares from each sheet, and place them on a lined baking sheet. Brush with melted butter and sprinkle a touch of salt. Bake for 12-14 minutes until puffed and golden, then allow to cool before slicing.

While the puff pastry is baking, grill your seasoned chicken breasts for about 6-8 minutes per side until cooked through. Let the chicken cool, then chop it into small pieces. In a large bowl, combine chopped chicken, diced celery, halved grapes, chopped onion, and mayonnaise, stirring until evenly coated.

Once puff pastry shells have cooled, slice each in half to create top and bottom. Spoon 2-3 tablespoons of chicken salad filling into the bottom halves, pressing the top halves back onto them, securing with a toothpick if desired.