Preheat the oven to 180 ºC/gas mark 4.
Roll out the pastry on a lightly ﬂoured surface into a rectangle about 3mm thick, then cut it into three equal rectangles, trimming appropriately. Place the pieces on a non-stick baking tray and brush with the egg wash. Bake for 15-20 minutes until well risen and golden brown. Remove from the oven and transfer to a wire rack to cool.
Melt the chocolate in a microwave or heatproof bowl over a pan of barely simmering water, making sure the bowl doesn’t touch the water, then remove from the heat. Add the hot water and whisk until fully emulsiﬁed. Scrape the vanilla seeds into the cream and whip until light and bulky.
Prepare the icing by mixing the icing sugar with enough lemon juice and water to achieve an easily spreadable paste (remember that the cooked pastry is very delicate and breaks easily) with a good balance of acidity.
Take the cooled sheets of pastry and trim if necessary to give three equal rectangles. Spread the icing over one sheet.
Spread half the vanilla cream on a second sheet then drizzle on half of the chocolate sauce. With the end of a knife gently nuzzle the chocolate into the cream. Repeat with the third pastry sheet then place on top of the second. Finally top with the iced pastry sheet and serve.
This is a devil to cut so you may prefer to make individual mille feuilles by cutting the sheets into smaller rectangles after baking then proceeding as above for each.
The time quoted above is an estimate of the time needed until the recipe is ready to eat, allowing for preparation, baking and resting.
As you might expect from Green & Black's, this recipe is indulgent and should be enjoyed as a treat..