Thaw the puff pastry either in the refrigerator overnight or at room temperature for about four hours until pliable. Cut into 36 squares, arrange them on a baking sheet lined with parchment paper. Brush with melted butter, sprinkle with salt, and bake at 400°F for 12-14 minutes until golden brown. Let cool before cutting in half.

While the puff pastry bakes, grill seasoned chicken breasts until they reach 165°F, about 6-7 minutes per side, then chop them into small pieces. In a mixing bowl, combine diced celery, halved grapes, chopped onion, and mayonnaise or Greek yogurt, stirring gently.

Once the pastry has cooled, take the bottom half of each puff and spoon about 2-3 tablespoons of the chicken salad filling onto it. Press the top half of the puff pastry on each filled bottom to secure. Arrange them on a serving platter.