Pre-heat the oven to 180C/350F degrees. Spread the hazelnuts out across a baking tray with a lipped edge and toast them for roughly 8 minutes until they are just golden. Remove from the oven to cool.
Meanwhile, in a medium, heavy bottomed saucepan bring the milk, butter and salt to the boil over a high heat, stirring until all the butter is dissolved. Remove from the heat.
Stir in the flour - don’t worry if it looks a bit of a mess - and return to a medium low heat. Beat vigorously for 3 minutes until the flour has cooked and the mixture is steaming. Again, remove from the heat.
Gently beat the eggs. Once the mixture has cooled enough so that it no longer steams, gradually beat in the egg until you have a smooth mixture. You want it to be just stiffer than the dropping consistency of a cake batter: add too much liquid and your choux won’t rise; if you need to, add a little more milk. If you happen to have egg whites in the fridge, use them instead and you’ll get a crisper choux.
Scrape the dough into a disposable piping bag and cut a cm off the end. Pipe 18 rounds onto two baking trays lined with baking parchment about 2/3 of the size you wish for the final choux to be, keeping space around them to allow them to grow without sticking together.
With a wet finger, gently flatten the peaks of the choux.
Bake in the oven for at least 15 minutes. The buns should have grown, puffed and turned golden. The longer you can keep them in the oven the better, but don’t let them burn !
The moment you remove the buns from the oven use a cake tester off the end of a barbecue skewer to poke a discreet hole in each bun to let the hot air escape and to prevent soggy choux.