Prepare homemade rough puff pastry dough through 2nd refrigeration or thaw store-bought puff pastry according to package directions.

In a heatproof bowl, whisk the egg yolks and cornstarch together. If dry, add a few drops of milk to combine. Set aside.

Combine milk and sugar in a saucepan over medium heat, whisking until sugar dissolves. Bring to a gentle simmer and remove from heat.

Gradually pour warm milk mixture into egg yolk mixture while whisking constantly. Strain back into saucepan.

Return saucepan to medium heat and whisk constantly until thickened to custard consistency, about 1?2 minutes. Remove from heat and whisk in butter, vanilla extract, vanilla bean seeds, and salt. Cool for 10 minutes.

Transfer pastry cream to a bowl, cover with plastic wrap directly on the surface, and refrigerate for at least 3 hours.

Preheat oven to 400?F (204?C) and line a 12x17-inch baking sheet with parchment paper.

Roll pastry dough into an 11x16 inch rectangle. Transfer to lined baking sheet, dock with a fork, and cover with parchment. Place a second baking sheet on top with weights.

Bake for 20 minutes. Remove top baking sheet and parchment; bake for an additional 7-9 minutes until golden brown. Cool for 20 minutes.

Whip the cold heavy cream until medium peaks form. Fold in chilled pastry cream and refrigerate for at least 1 hour.

Trim edges of cooled pastry and cut into 15 squares.

In a bowl, whisk together confectioners? sugar, milk, and vanilla for the icing. Melt chocolate and transfer to a piping bag.

Ice one pastry square, pipe chocolate lines across, and create a chevron design with a toothpick. Repeat for remaining squares.

Assemble pastries by piping cream filling on undressed pastry squares, layering them, and topping with decorated squares. Refrigerate for at least 30 minutes before serving.