Chilling time: 2 hours + 30 minutes + 1 hour + 3 hours.
1 . For the crust, sift the flour, cocoa, sugar and salt into a large mixing bowl or in the bowl of a stand mixer fitted with the paddle attachment. Add the butter plus mix or cut in with a cut a pastry cutter, if mixing by hand, until a rough crumbly texture, and no large bits of butter are visible. Whisk the egg yolk and water together and add this to the bowl, plus blend until the dough just comes together. Shape this into a disc, wrap in plastic wrap and chill for at least 2 hours.
2 . Lightly knead the particular dough a minute on a lightly floured surface, to soften it just slightly. Roll the dough out into a circle just under 6 mm (¼ in) thick. Line a 23 cm (9 in) fluted removable-bottom tart pan with the pastry and press in, trimming away any excess. Chill this for at least 30 minutes.
3. Preheat the oven to 160°C (325 °F). Dock the bottom of the pastry with a fork (this prevents the crust from developing air pockets) and bake for about 20 minutes, until the pastry loses its shine at the centre of the shell. Cool the pan on a rack and keep the oven at 160°C (325 °F).
4. For that truffle layer, bring the cream to a boil in a small saucepan (watch it carefully – it will boil up quickly) and pour this over the chopped chocolate. Let this sit for 30 seconds then slowly stir it until smooth. Whisk in the sugar. Break the egg into a small dish and stir it a little before adding to the chocolate along with the vanilla and whisking within well. Pour the truffle filling into the cooled shell and bake for 10 minutes (there will be no visible difference – it will still seem fluid). Cool the particular tart on a rack and then chill for at least an hour.
5. For your fluffy layer, whisk the eggs, sugar, water plus salt in a metal bowl placed over a pot of gently simmering water and whisk vigorously until the mixture holds a ribbon when the whisk is lifted, about 8 minutes. Pour this directly over the chopped milk chocolate and whisk until smooth, returning to the heat if needed, to make sure there are no unmelted chocolate bits. Set aside to cool to room temperature (this won’t take too long).
6. Using electric beaters, or placing the chocolate base into the bowl of a stand mixer fitted using the whip attachment, add the particular butter and whip till it is fully incorporated. In a separate bowl whip the cream and vanilla to a soft peak and fold into the chocolate until evenly combined. Spread this on the tart (it will be soft but not fluid) and chill for at least 3 hours before serving. Use a vegetable peeler to make chocolate shavings and sprinkle them within the tart.