Puff pastry:In the mixer bowl, combine the flour, sugar, and salt.Add the water and mix until the dough forms a ball.Cover the bowl with a cloth and let it rest for 20 minutes.Flour the work surface and roll out the dough into a rectangle, thicker in the center.Place the butter in a single, flattened mass in the center of the dough.Fold the dough over the butter and seal the edges to enclose the fat.Then, roll out the dough into a rectangle 1/2″ (1 cm) thick, three times as long as it is wide.Fold the rectangle into thirds, folding the first third towards you and the third over the top.Turn the dough 1/4 turn to the right.Immediately give it a second turn: roll out the dough into a rectangle, fold it into thirds, give it 1/4 turn to the right, and let it rest in the refrigerator for 20 minutes.The classic puff pastry is made with 6 turns (3 series of 2 turns with a rest every 2 turns).After 6 turns, the dough is ready to use.If you prepare the dough the day before, give it the last two turns when you’re ready to use it.Pastry cream:Heat the milk in a saucepan.In a bowl, combine the egg yolks, sugars, and cornstarch.Mix the preparation with the hot milk and return it to the saucepan.Bring to a boil, whisking constantly, until it thickens.Remove from heat, add the butter, and mix well.Cover the saucepan with plastic wrap and let it cool completely.Assembly:Roll out the dough into a rectangle 9″ x 13″ (23 cm x 33 cm) and 1/4″ (0.5 cm) thick.Place the dough on a buttered baking sheet, brush it quickly with a brush, and prick it with a fork.Bake in a preheated oven at 400 F (200 C) for 20 minutes.Divide the baked puff pastry into three equal strips.Place one strip of dough, then spread with half of the pastry cream.Repeat the operation once, ending with the puff pastry.Glaze:Mix the powdered sugar and milk, then coat the top of the last strip of dough with it.Using a piping bag, draw lines of melted chocolate on the glaze, then create designs with the tip of a knife.Refrigerate, then cut into slices.