Chocolate and chestnut éclairs

Method

1. First make the icing. Put the gelatine in a bowl of cold water to soften and set aside. Put the sugar, cocoa and cream in a pan along with 125ml water, bring to the boil and cook, whisking, until the mixture thickly coats the back of a teaspoon. Remove from the heat. Squeeze the excess water from the gelatine sheets, add to the pan and stir 
until dissolved. Set aside to cool.
2. Preheat the oven to 200°C/fan180°C/gas 6. Line a baking sheet with baking paper. Sift the flour, sugar and a pinch of salt into a heap on a separate sheet of baking paper.
3. Heat 220ml water and the butter in a large saucepan over a medium 
heat until the butter has melted. Increase the heat to bring it to 
a vigorous rolling boil.
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4. Using the baking paper as a funnel, quickly tip the flour mixture into the water and butter, then remove from the heat and beat vigorously with a wooden spoon until the mixture forms a thick 
paste that comes away cleanly 
from the sides of the pan.
5. Gradually add the beaten eggs, beating between additions until you have a smooth, glossy mixture. (You may not need all the eggs – add just enough to give the choux pastry a thick dropping consistency.)
6. Put the choux pastry in a piping bag with a large nozzle and pipe thick lines about 14cm long onto the lined baking sheet. Bake in the oven for 25-30 minutes until risen, golden and crisp. Remove from the oven, pierce in a couple of places along the top of each to allow steam to escape, then cool on a wire rack.
7. To make the filling, heat the milk and cream in a pan until boiling, then simmer for a few minutes. Remove from the heat.
8. Whisk the egg yolks, sugar and flour in a bowl, then pour over the cream mixture and whisk together. Pour the mixture into the pan and bring back to the boil, whisking constantly for 2-3 minutes until really thick. Pour back into the bowl and set aside.
9. Put the chopped chocolate in a small heatproof bowl set over a pan of simmering water. When the chocolate has melted, remove from the heat and cool a little. Add the cooled chocolate to the rest of the filling mixture, then add the mashed chestnut purée and icing sugar to the bowl. Stir well. Set aside with cling film covering the surface to prevent a skin forming.
10. To finish, spoon the filling into a piping bag with a medium star-shaped nozzle. Cut the choux buns in half, then pipe in a generous amount of the filling. Reassemble the eclairs, then drizzle with the icing. Chill. Remove from the fridge 15 minutes before you want to eat them, then serve.