Roll puff pastry sheet on a lightly floured surface into a 14-inch square. Cut puff pastry into 2-inch squares. Grease cups of 1-3/4-inch muffin pans and line pastry squares, allowing corners to extend above cups. Beat together egg yolk and water in small bowl. Brush pastry cups with egg yolk mixture.
Bake in a 375 degree F oven for 12 to 15 minutes or until golden brown. (Pastry will puff, leaving small indentations.)
Meanwhile, combine smoked trout, sour cream, lemon juice, horseradish, and honey mustard in a medium mixing bowl or food processor bowl. Beat with an electric mixer, or cover and process until almost smooth.
Fit a pastry bag with a large star tip and fill with smoked trout mixture. (Or, fill a self-sealing plastic bag with mixture, and snip 1/4 inch off one corner.) Pipe filling on top of each pastry. Top each appetizer with a sprig of dill, if desired. Spoon 1/8 teaspoon caviar onto each. Refrigerate till serving time. Makes about 48.