Prick the rectangles of puff pastry with a fork so that they do not puff up while baking.

Place them on a baking tray covered with baking paper and cover them with a silicone mat (or another sheet of baking paper and a heavy baking tray). This will prevent them from puffing up too much.

Bake them in a preheated oven at 200°C – 400°F for 15 minutes, then remove the top tray and baking paper, sprinkle them with icing sugar and put them back in the oven (uncovered) to brown and caramelize for another 5 to 10 minutes. Keep an eye on them at this stage as the sugar can burn.

Remove from the oven and leave the puff pastry rectangles to cool down on a wire rack.

Once the pastry has cooled, you can assemble your mille-feuille.

Put the crème pâtissière in a piping bag filled with a big round nozzle (or cut a hole in a reusable bag and use it without a nozzle).

Pipe enough crème pâtissière to cover 4 puffy pastry rectangles.

Top them with another 4 rectangles. Press down lightly with your hands to ensure that they stick to the filling.

Make another layer by piping enough crème pâtissière to cover the 4 puffy pastry rectangles and top them with the last 4 rectangles. Press down lightly with your hands to ensure that they stick to the filling.

Put them in the fridge and prepare the icing.

Melt the chocolate and transfer it to a piping bag (or plastic bag with the end snipped), fitted with a small round nozzle. Keep it in a glass or other tall container.

To make the icing, whisk the egg white, the lemon juice and icing sugar with an electric mixer until smooth. The mixture should be thick enough to leave trails on the surface. If it’s too thin, whisk in a bit more icing sugar.

Once ready, take the pastries out of the fridge, immediately pour the icing over the top of the mille-feuille and spread evenly (or remove the top puff pastries and dip them in it).

Still working quickly, pipe thin chocolate lines along the width of your pastry sheet (see below).

Now take a toothpick or a sharp knife and lightly draw it down (from top to bottom) through the rows of chocolate.

Chill for a couple of hours to give the icing the time to set and for all the flavours to combine.

Serve cold or at room temperature.