Step 1

Preheat the oven to 220°C, gas mark 7. Line a baking tray with baking parchment. Sift the flour into a bowl with a pinch of salt. Put 50g of the butter in a medium saucepan with 125ml water and set over a medium heat until melted, then bring to the boil. Take the pan off the heat, then tip the flour into the melted butter mixture and beat with a wooden spoon until the mixture is starting to come away from the sides of the pan (about 1 minute).

Step 2

Tip into a mixing bowl, then add the beaten egg, a little at a time, beating well between each addition to form a shiny, glossy mixture (you may not need to use it all; keep adding just until the mixture falls off an upturned spoon in 3 seconds). Use a large spoon to dollop 6 mounds of the choux bun mixture onto the baking tray. If the tops are bumpy, dip a finger in water and use to smooth them out.

Step 3

Bake for 15 minutes, then reduce the oven temperature to 180°C, gas mark 4, and bake for a further 10-15 minutes until golden and puffed up. Use a serrated knife to carefully cut the top 1⁄3 off each bun, then return the tops and bases to the oven, cut-sides up, for a further 2-5 minutes to dry out. Transfer to a wire rack to cool. Tip the hazelnuts onto a baking tray and roast for 4-5 minutes until golden; allow to cool, then roughly chop.

Step 4

To make the filling, melt 100g chocolate in the microwave or over a pan of simmering water, then set aside to cool. In a mixing bowl, use an electric hand mixer to beat the mascarpone, double cream, cooled chocolate, coffee, alcohol and a pinch of salt until light and fluffy (about 3-5 minutes). Spoon into a piping bag fitted with a star nozzle, then generously pipe the mixture into the base of each choux bun. Scatter over the hazelnuts. Sieve the cocoa powder over the choux bun tops, then rest these on the filling. The empty choux buns and mascarpone filling can be made a few hours ahead; cover and store the mascarpone mixture in the fridge and fill the buns just before serving.

Step 5

When you are ready to serve, put the remaining 100g chocolate, 125ml hot water and the caster sugar in a pan set over a medium heat until melted, stirring frequently. Bring to the boil and cook until the mixture thickens and becomes syrupy (8-10 minutes), then beat in the remaining 15g butter. Divide the filled buns among 6 plates and pour over a generous amount of the chocolate sauce to serve.