1. For pastry, defrost according to package instructions. Unfold the two pastry sheets in each box and separate each along the folds into three rectangles. Place the rectangles on parchment lined baking sheets and dock with a fork. Top with additional parchment and weight with another baking sheet. Bake until golden according to package. Cool on racks. Trim with a serrated knife to create straight edges.
2. For custard, mix the sugar, cornstarch and salt in a saucepan and gradually add the milk. Cook over medium heat, stirring constantly, until thickened and boiling. Stir and boil for one minute. Slowly add some of the mixture into the egg yolks to temper. Stir egg mixture into the remaining hot milk. Boil and stir for one minute. Off the heat, stir in the butter and vanilla. Chill with plastic wrap directly on the surface until cold.
3. Place the chocolate pieces and heavy cream in a bowl set over 1-inch of boiling water. Heat and stir until melted and smooth. Cool to thicken.
4. To assemble, reserve ½ cup fresh raspberries for garnish. On a sheet pan, spread half the custard over two of the sheets of pastry, top with a single layer of raspberries. Place another sheet of pastry over the raspberries and repeat layering of custard and raspberries. Top with remaining sheets of pastry. Place the pan in the refrigerator to set for 10 minutes. Gently slice the layers of each strip with a serrated knife into 5 rectangular pastries. Dust with powdered sugar, drizzle with chocolate and top with reserved raspberries.