Divide puff pastry in half lengthwise. Roll each half into a 15- x 6-inch rectangle. Sprinkle two baking sheets with water; shake off excess water. Carefully transfer pastry to baking sheets.

Prick pastry well with a fork. Trim sides with a sharp knife, if necessary, to make edges even. Freeze 10 minutes. Bake at 425°F for 15 minutes or until pastry is puffed and golden brown. Gently remove pastry from baking sheets with spatulas, and cool on wire racks.

Trim sides of pastry again, if necessary, to make edges even. Carefully split each layer in half horizontally, using a long serrated knife. Set aside the prettiest bottom layer for top of Napoleon. Place one pastry strip on serving platter, browned side down; spread evenly with one-third Napoleon Cream. Repeat layering twice. Top with reserved bottom layer, browned side up.

Combine powdered sugar, vanilla, and water; spoon glaze over top pastry layer, spreading evenly.

Combine chocolate and shortening in top of a double boiler; bring water to a boil. Reduce heat to low; cook until chocolate melts. Let cool slightly; then spoon into a decorating bag fitted with metal tip No. 2. Pipe 5 lengthwise strips of chocolate evenly across top of glaze. Pull a wooden pick crosswise through chocolate at 3/4-inch intervals, reversing the pulling direction each time. Refrigerate 30 minutes before serving. To serve, cut into 1 1/2-inch crosswise slices using a serrated knife.

Napoleon Cream

Combine first 3 ingredients in a heavy saucepan, stirring until cornstarch is blended. Stir in milk. Cook over low heat, stirring constantly, until mixture is thickened.

Beat egg yolks until thick and lemon colored. Gradually stir about one-fourth of hot mixture into yolks; add to remaining hot mixture, stirring constantly. Cook, stirring constantly, until mixture thickens. Remove from heat, and stir in vanilla. Cover filling with a sheet of wax paper, and chill thoroughly. Gently fold whipped cream into chilled filling.

Basic Puff Pastry.