Begin by making the sugar syrup, put the water, sugar and vanilla extract in a saucepan. Bring to the boil, and cook for 2-3 minutes.
Take off the heat and leave to cool.
Next make the chestnut mousse, Soak the gelatine in a bowl of ice cold water for a few minutes until soft.
Place the unsweetened chestnut puree in a pan with the syrup and bring to the boil.
Take off the heat and add the soaked gelatine, squeezing all of the water out before adding it.
Mix well and pass through a fine sieve, then leave to cool to room temperature.
Mix in the dark rum and then fold in the semi-whipped cream.
Fill the small hemisphere moulds that are 3cm diameter and freeze for at least 2 hours.
Next make the chocolate hazelnut discs, melt the chocolate and mix in the chopped hazelnuts, spread the mixture onto a silicone sheet and leave to set hard.
Take a 3cm round cookie cutter and stamp 12 circles, store them in an airtight container till you are ready to use them.
To make the sable breton, put the plain flour, salt and baking powder in a bowl and mix together. Put this to one side.
Put the sugar and egg yolks in the bowl of a stand mixer and beat until light and pale. Add the softened butter and mix, the mixture may look curdled, but this is fine.
Then add the flour mixture and mix on a low speed until combined. The dough will not completely come together, tip the dough out onto a sheet of baking parchment and knead it into a ball.
Place another sheet of baking parchment on top and roll the pastry out to a thickness of 5mm. Place the dough in the freezer for 20 mins to rest.
Preheat the oven to 180C/160C fan/gas 4 and take the pastry from the freezer, peel off the top layer of baking parchment. Place the pastry in one piece on a baking tray and bake in the oven for 12 minute until it becomes golden.
As soon as the pastry is baked, take a 7cm round pastry cutter and stamp out 6 circles from the pastry. Transfer the circles to a rack and leave to cool.
To make the chocolate mousse melt the chocolate in a bain-marie until it reaches 45C.
Whisk the egg yolks in an electric mixer fitted with the whisk attachment.
Meanwhile put the sugar and water in a saucepan and bring to the boil. Heat until it reaches 121C, then slowly pour the sugar syrup over the egg yolks and continue to whisk to a full sabayon. Continue to whisk the mixture until it is cool.
In a separate bowl whip the cream until it reaches the ribbon stage, then fold the sabayon into the cream.
Carefully fold one third of this mixture into the melted chocolate. Fold in the remaining cream.
Next assemble the domes, take the 7cm diameter silicone moulds and spoon some chocolate mousse into the mould, using a stepped palette knife, smooth the mousse up the sides of the mould.
Place in the frozen chestnut mousse semi-spheres and push it down a little, place a chocolate disc on top and then fill the mould with the remaining chocolate mousse. Repeat this with the remaining 5 moulds and place them in the freezer for at least 2 hours.
Finally make the dark chocolate glaze, soak the gelatine in a bowl of cold water for a few minutes until soft. Squeeze out the excess water.
Put the water and sugar in a saucepan, bring to the boil, then continue to simmer over a low heat for 2-3 minutes.
Add the sifted cocoa powder and the cream, mix thoroughly.
Bring back to the boil and simmer for 4-5 minutes.
Take the pan off the heat and add the pre-soaked gelatine and stir until dissolved.
Strain then leave to cool.
When the domes are frozen, take them out of the moulds and place them on a cake rack with a baking tray underneath, take the glaze and warm it to around 35C-40C and then pour it over the mounds until completely covered.
When the glaze has finished dripping place a dome on top of a sable breton on the flattest side. Take the remaining 6 chocolate discs and place them on top of the domes at an angle. Decorate with gold leaf.