Prepare and chill Quick-Method Puff Pastry Dough. Line a baking sheet with parchment paper or plain brown paper.
On a lightly floured surface roll the puff pastry dough into a 16x12-inch rectangle. Using a sharp knife, cut pastry lengthwise into twelve 16x1-inch strips. (If using purchased pastry, unfold and roll each sheet into a 12x10-inch rectangle. Cut each sheet into twelve 10x1-inch strips. Press 2 strips of purchased pastry together at ends to make a long strip; repeat with remaining strips.)
Wrap each strip of dough around a well-greased cream horn mold or 3/4-inch-wide cannoli tube. Overlap layers slightly and press gently. Place 1 inch apart on the prepared baking sheet. In a small bowl beat egg white and water using a fork; use a brush over pastry. Sprinkle with the sugar.
Bake in a 425 degree F oven for 15 to 20 minutes or until golden for Quick-Method pastry (12 to 15 minutes for purchased pastry). Transfer to a wire rack. While still warm, slightly twist the molds and remove from pastry horns. Cool on a wire rack.
Prepare Mocha Pastry Cream; chill. Spoon cream mixture into a pastry bag fitted with a large star tip (about 1-inch opening). Pipe cream into each horn. If desired, chill up to 1 hour. Makes 12 cream horns.
Mocha Pastry Cream
In a heavy saucepan stir together sugar, all-purpose flour, and salt. Gradually stir in half-and-half or light cream; semisweet chocolate, chopped; and instant coffee crystals. Cook and stir over medium heat until thickened and bubbly. Cook and stir 1 minute more. Gradually stir about half of the hot mixture into 4 beaten egg yolks. Return all of the yolk mixture to the saucepan. Bring to a gently boil; reduce heat. Cook and stir for 2 minutes. Remove from heat. Cook and stir for 2 minutes. Remove from heat; stir in vanilla. Transfer to a bowl. Cover surface with plastic wrap. Cool slightly. Chill 2 to 3 hours or until cold (do not stir). In a chilled small mixing bowl beat whipping cream until soft peaks form. Gradually fold whipped cream into cooled pastry cream; chill well before using.