In large bowl, stir together mayonnaise, mustard, lemon juice, and salt, if desired, until well-blended. Gently stir in eggs and carrots. Set aside. On lightly floured surface, roll 1 sheet of the puff pastry into 15 x 15-inch square. Brush with some of the beaten egg. Cut rolled pastry in half. Place one half of the pastry on 15-1/2 x 12-inch baking sheet. Dollop scant tablespoons of the reserved egg salad mixture onto pastry on sheet in 3 lengthwise rows of 5 each, about 1 inch apart. Top with remaining half of rolled pastry. Press outside edges together to seal. Gently, but firmly, press pastry together in lines between dollops of salad. Brush with more of the beaten egg. Cut into squares at centers of pressed lines. Bake in preheated 375ºF oven until puffed and golden brown, about 30 minutes. Repeat with remaining sheet of puff pastry, egg salad mixture, and beaten egg. Garnish with carrot leaves, if desired. If prepared in advance, reheat in preheated 375ºF oven about 7 to 8 minutes, if desired. Makes 8 servings of 3 to 4 puffs each.

*To hard-cook, put eggs in single layer in saucepan. Add enough tap water to come at least 1 inch above eggs. Cover and quickly bring just to boiling. Turn off heat. If necessary, remove pan from burner to prevent further boiling. Let eggs stand, covered, in the hot water 15 minutes for large eggs. (Adjust time up or down by about 3 minutes for each size larger or smaller.) Immediately run cold water over eggs or put them in ice water until completely cooled. To remove shell, crackle it by tapping gently all over. Roll egg between hands to loosen shell, then peel, starting at large end. Hold egg under running cold water or dip in bowl of water to help ease off shell.