To make the chocolate sable breton, put the plain flour, fleur de sel, baking powder and cocoa 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 minutes, don't allow the pastry to colour at all at the edges, you want it to remain chocolate coloured.
As soon as the pastry is baked, take a 6cm round pastry cutter and stamp out 12 circles from the pastry. Transfer the circles to a rack and leave to cool.
To make the chestnut cream, take the chestnut puree and beat it with the butter in the bowl of a stand mixer. The two ingredients need to be the same temperature so they mix together, then add the icing sugar and bring it all together. You should end up with a smooth buttercream, place it in the fridge a little to firm up if it's too soft.
Place the cream in a bowl along with the amaretto and whip until it forms stiff peaks, put the cream in a disposable piping bag and leave in the fridge to stay cool.
When the sables are cooled, pipe blobs of amaretto cream on top of the sable, pipe upwards and leave a gap of around half a centimetre for the chestnut cream.
Beat the chestnut cream to make sure it's fully combined and place the cream in a piping bag fited with a small straight nozzle. Pipe the chestnut cream around and over the whipped cream, making sure you cover it entirely.
Dust the Mont Blancs with icing sugar and decorate the top with gold leaf.