Unfold thawed puff pastry on a clean surface and cut each sheet into 36 squares. Place on a lined baking sheet. Brush with melted butter and sprinkle with salt. Bake at 400°F for 12-14 minutes, until puffed and golden.

Season chicken breasts and grill for 6-7 minutes per side. Cool and chop into small pieces.

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

Slice each puff pastry square in half. Fill the bottom half with 2-3 tablespoons of filling. Cap with the top half and secure with a toothpick if needed.

Serve immediately or store in the fridge until your gathering.