Add the milk, butter and 125ml water to a medium saucepan over a medium heat. Heat until the butter has melted, then bring it to a rolling boil.
Remove from the heat and immediately add the flour and sugar, beating with a wooden spoon until the mixture is smooth, comes together and easily comes away from the side of the pan. Transfer to a large plate and spread out, leaving until cool enough to handle by hand. Once ready, transfer back to the pan and beat in the eggs, one by one, until smooth and glossy.
Preheat the oven to 220°C/gas mark 7. Mark two large pieces of baking paper with two rows of lines, 9cm in length and spaced evenly part. Turn them over and line two baking trays with the paper.
Using a nozzle with a width just over 1cm, fill a piping bag with the dough and pipe on to the trays, using the lines as guides. Transfer the trays towards the top of the oven and bake for 25 minutes until golden brown. Once baked, transfer to a wire rack and immediately, but carefully, slice down the side allowing any steam to escape while they cool, so they don’t get soggy. Set aside to cool completely.
Now start on the crème pat. Warm the milk in a large saucepan until it starts to get steamy, but doesn’t boil. Set aside. In a large bowl, whisk together the sugar and flours. Add the egg yolks and whisk them into the dry ingredients, forming a thick paste.
Pour a little of the hot milk into the egg yolk mixture and whisk to combine. Gradually add the rest of the milk slowly into the eggs, whisking continuously. When all the milk has been added to the eggs, pour everything back into the saucepan. Set a sieve over a bowl and place near the stove.
Set the pan back over a medium heat and whisk constantly until it starts to thicken. This will take a few minutes. Once it starts to bubble (you will have to pause briefly between whisking to check) remove from the heat and beat in 2 tsp vanilla extract. Pour through the sieve to remove any cooked egg. Spread out on a plate, cover with clingfilm and leave to cool.
Heat the oil in a large deep saucepan with a lid, over a medium heat. Add 2 popcorn kernels. As the corn starts to turn, add the remaining kernels and cover. When the corn starts to pop, shake the saucepan gently. Remove from the heat when the corn stops popping. Transfer to a large bowl, discarding any unpopped corn and add the peanuts.
Add the butter, sugar and honey to a medium saucepan and stir over a medium heat for 6-8 minutes until everything is melted and combined. Bring to the boil for 5 minutes, without stirring, giving the saucepan an occasional shake until the mixture takes on a rich copper colour.
Remove from the heat. Add the salt, bicarbonate of soda and remaining 1 tsp vanilla extract and stir together – it will bubble slightly. Working quickly, pour ¼ of the caramel over the popcorn and stir until covered and glistening. Transfer to a greaseproof paper-lined baking tray and bake in a 125°c/gas mark ½ for 30 minutes, stirring halfway through to break up any clumps.
Meanwhile, place the remaining caramel back over a low heat and gradually stir in the cream until smooth. Let it cool, then beat ½ together with the crème pat mixture until smooth. Set aside. Mix the remaining caramel with the fondant icing sugar. If it’s too thick, add 1-2 tbsp hot water so it has a spreadable, but not runny, consistency.
Fill the éclair shells with the caramel crème pat using a icing bag and 1cm nozzle. Use a palette knife to spread the icing on to each of the eclairs. Top with the popcorn and a few flakes of sea salt, if desired.