Unfold the puff pastry sheet with care. Using a pizza cutter, slice along the fold marks to create 3 strips. Cut each strip in half crosswise, then continue dividing the pastry until you have evenly sized squares for baking.
Place the pastry squares on a baking sheet lined with a Silpat or other nonstick liner. Brush the tops generously with the melted butter, then scatter the coarse salt lightly over each piece.
Bake in a preheated 400°F oven for 12 to 14 minutes, or until the pastry is puffed and deeply golden. Remove from the oven and let the squares cool completely before slicing each one horizontally into a top and bottom half.
Season the chicken breasts with salt and black pepper. Grill until fully cooked through, then set aside to cool. Once cooled, chop the chicken into small, bite-sized pieces.
In a mixing bowl, combine the chopped chicken, finely diced celery, quartered grapes, minced onion, mayonnaise, coarse salt, and Creole seasoning. Stir thoroughly until the mixture is evenly blended and well coated.
Spoon a generous portion of the chicken salad onto the bottom half of each pastry square. Set the top half over the filling to form a small sandwich.
For serving, secure each puff with a decorative toothpick if desired, then arrange them neatly on a platter.