Unfold pastry on a lightly floured surface. Lightly brush pastry with egg white. Sprinkle with coarse sugar. Cut the sheet of puff pastry into 25 squares. Place squares 2 inches apart on an ungreased baking sheet. Bake pastry in a 400 degree F oven about 13 minutes or until golden. Transfer to wire racks and cool. Place in an airtight container and store at room temperature up to 24 hours.
For pastry cream, stir together granulated sugar, flour, and salt in a heavy medium saucepan. Gradually stir in half-and-half or light cream and white chocolate. Cook and stir over medium heat until thickened and bubbly. Cook and stir for 1 minute more.
Gradually stir about half of the hot mixture into the beaten egg yolks. Then return all of the egg yolk mixture to the saucepan. Bring to a gentle boil. Reduce heat; cook and stir for 2 minutes. Remove from heat. Stir in vanilla. Cover surface of pastry cream with plastic wrap. Cool just until warm without stirring (about 1-1/2 hours).
Beat whipping cream in a small bowl with an electric mixer on medium-high speed until soft peaks form. Beat in lemon curd just until combined. Fold whipped lemon cream into the warm pastry cream. Cover and chill for at least 1 hour or up to 6 hours.
To assemble, split pastry squares into two layers. Pipe cream mixture onto bottom pastry layers using a pastry bag fitted with a 1/2-inch star or plain tip, or spoon cream mixture onto bottom pastry layers. Add pastry tops. Arrange pastries on a platter. Makes 25.