Combine 1 1/2 cups sugar, 1/2 cup cornstarch, and salt. Add milk, stirring well. Cook over medium heat, stirring constantly, until thick. Beat egg yolks until thick and pale. Stir one-fourth of hot mixture into yolks; add to remaining hot mixture, stirring constantly. Bring to a boil; cook, stirring constantly, 1 minute or until thick. Remove from heat; stir in vanilla. Let stand at room temperature 30 minutes. Cover and chill.
Roll pastry to remove fold lines. Cut into 8 squares. Place on baking sheet; bake at 350° for 15 minutes. Let cool. Cut each square in half horizontally.
Place strawberries in an electric blender; cover and process until smooth. Press through a sieve; discard pulp. Combine strawberry puree and remaining 1 tablespoon cornstarch in a saucepan; stir. Add jelly; cook over medium heat until thick. Cook, stirring constantly, 1 minute. Remove from heat; stir in Cognac.
Place bottom halves of pastry squares on plates. Fold whipped cream into custard; spoon onto pastry squares. Top with remaining halves. Sprinkle with powdered sugar and chocolate. Spoon strawberry sauce in small circles onto plates; pull a wooden pick through circles. Garnish with berries, if desired.