The mousse filling should be made several hours in advance.
First, melt the chocolate in a heatproof bowl set over a pan of barely simmering water (make sure the base of the bowl doesn't touch the water). Then beat the egg yolks into the chocolate and allow the mixture to cool. Now, using an electric hand whisk, whisk the egg whites to the soft peak stage. Stir 1 tablespoon of the egg white into the chocolate mixture to loosen it, then carefully fold in the remainder. Cover the bowl and chill the mousse in the fridge for about 3 hours.
To make the choux pastry, you are going to need to 'shoot' the flour quickly into the water and melted butter, so begin by cutting out a square of baking parchment, fold to make a crease, then open it out again. Sift the flour straight on to the square of parchment and add the sugar. Next, put 5 fl oz (150 ml) of cold water in a medium saucepan, together with the pieces of butter, then place the saucepan over a moderate heat and stir with a wooden spoon. As soon as the butter has melted and the mixture comes up to the boil, turn off the heat immediately, as too much boiling will evaporate some of the water.
Then tip in the flour – all in one go – with one hand, while you beat the mixture vigorously with the other. You can do this with a wooden spoon, although an electric hand whisk will save you lots of energy. Beat until you have a smooth ball of paste that has left the sides of the saucepan clean – this will probably take less than a minute. Then beat in the beaten eggs – a little at a time, mixing in each addition thoroughly before adding the next – until you have a smooth, glossy paste.
At this stage, hold the greased baking sheet under cold running water for a few seconds, and tap it sharply to get rid of excess moisture. This will help create a steamier atmosphere, which in turn helps the pastry to rise. Place rough dessertspoons of the choux pastry on the greased and dampened baking sheet and bake on a high shelf in the pre-heated oven (200C) for 10 minutes. After that, increase the heat to gas mark 7, 425ºF (220ºC) and bake for a further 20-25 minutes until the buns are nicely brown and puffy. Pierce the side of each one and put them back in the oven for a further 2-3 minutes to crisp up, then cool on a wire cooling tray. It's best not to put the filling in until about an hour before serving.
They are best made and eaten on the day but if you want to get ahead you and make, cool and freeze but they will go soft but they can be crisped up in the oven (from frozen) before you fill them. Then keeping the heat at its lowest, allow the chocolate to melt slowly - it should take about 5 minutes to become smooth and glossy. Then remove it from the heat, give it a good stir and leave to cool for 2-3 minutes. Next, stir the crème fraîche into the chocolate and spread some of the topping over each bun and finally, sprinkle with chopped hazelnuts.
Note: this recipe contains raw eggs.
Pre-heat the oven to gas mark 6, 400°F (200°C).
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