Put the egg whites into the bowl of a stand mixer and whisk them until they form soft–medium peaks. Then, gradually add the caster sugar, a quarter at a time, making sure that it is well whisked before adding the next bit. Keep whisking to give shiny and stiff peaks and then set this aside. Some people say to hold the bowl over your head – if it doesn’t fall out, it’s ready!
Put the cocoa powder and icing sugar together in a bowl and use a balloon whisk to combine, getting rid of any lumps. Then tip this into the meringue and quickly fold together until smooth and uniform. Half-fill the piping bag with the mixture – filling halfway ensures that the mixture does not squidge out of the top when you are piping.
Line a baking sheet with baking parchment and use a blob of meringue in each corner to stick it down. Pipe thirty meringue kisses on the sheet, each about 2–3cm in diameter, then
Meanwhile, make the pastry. Put the butter and the icing sugar together in a stand mixer and cream together well using a paddle attachment. Then add the egg yolk and beat until combined, making sure you scrape the bowl occasionally. Add the flour and salt, mixing just so that the mixture is uniform. You do not want to over-mix your mixture or it will become very tough. The mixture will look more spreadable than usual pastry dough, but should come together when mixed. Once well mixed, scrape the dough out of the bowl, shape it into a ball and press it down a little to form a large, thick disc. Wrap well in cling film and pop in the fridge for a good hour (or more if you can), to allow the dough to firm up and for the gluten to relax.
When the pastry is firm, roll it on a lightly floured surface to a circle, about 33cm in diameter, with the thickness of half a £1 coin (about 1.5mm). Use this to line the tin, making sure to get it into the ‘corners’. I like to take a small ball of pastry, dip it in flour and then use this to gently ease the pastry into the tin. Try not to stretch the pastry as this may cause it to shrink. Use a sharp knife to cleanly cut the excess pastry from the edges. Pop the pastry case into the freezer for 10 minutes and
Cut a piece of baking parchment into a circle, about 35cm in diameter. Scrunch it up so that it will sit in the tin, and then open it out and place it on top of the pastry case. Fill the tin with baking beans, dried pulses or rice, making sure that they come right up the sides. Place the tin on a baking sheet and
Meanwhile, make the mousse. Melt the chocolate in the microwave using 30-second blasts, stirring well between each blast so that it does not burn, then set this aside to cool. Put the sugar into a small pan with the water and bring it to the boil, stirring until dissolved. Pop the thermometer in and continue to boil until the mixture reaches 121°C (252°F). Meanwhile, put the eggs into the bowl of a stand mixer. As the sugar nears 121°C (252°F), start mixing the eggs until they are thick and frothy. As soon as the sugar reaches 121°C (252°F), gradually add it to the eggs while beating on top speed, making sure that the solution does not touch the whisk attachment. Once all the sugar solution has been added, beat for about 10 minutes, until the mixture has cooled, is light and fluffy and has doubled in volume. Set aside.
When the tart case has been
Whisk the cream in a medium bowl until thick and just holding its shape, being careful not to over-whisk it. Pour the melted chocolate into the whisked egg white mixture and fold to combine, very gently so that you don’t knock all of the air out of it. Fold in the whipped cream until totally uniform, and set aside.
Once the pastry is cool, fill it with the mousse and pop it into the fridge for about 30 minutes to set a little. Then arrange the meringues on top, sprinkle with cocoa powder if you fancy it, and serve.