To make the craquelin, put the flour, sugar and butter in a bowl and rub with your fingertips, like a crumble mix. Bring together until the mixture forms a dough, pushing it together if it crumbles (it will be quite short). Roll between 2 sheets of baking paper using a rolling pin, until it’s around 2mm thick and approx 20x25cm, then chill.
Heat the oven to 180C/fan 160C/gas 4. Sift the flour into a bowl and add a pinch of salt. Put the sugar, butter and milk in a pan with 125ml water and bring to the boil. Take the pan off the heat, pour in the flour and beat the mixture until it turns into a smooth, thick dough that comes away from the side of the pan. Tip this into the bowl of a stand mixer and beat for 2 minutes to cool it down a bit (otherwise the eggs might cook when you add them), or beat well with a wooden spoon.
Add the eggs, a little at a time, continually beating. Don’t worry if the mixture looks as if it has split, it will come back together. Spoon the mix into a piping bag fitted with a plain round nozzle about 1½ cm wide and pipe éclairs about 12cm long onto a baking sheet lined with a non-stick baking mat or baking paper, spacing them well apart.
Wet your finger with water, and smooth down any piping peaks on the éclairs.
Cut the craquelin into strips 12cm long and as wide as the éclairs, put them on top of the piped éclairs and bake for 40-50 minutes or until the éclairs are dry and puffed. Cut one in half to check they’re completely dry in the middle. Leave to cool. For the filling, whip the cream with the icing sugar and vanilla to make soft peaks. Spoon into a piping bag with a plain, thin nozzle, about 2mm wide.
When the éclairs have cooled, use a skewer to pierce 3 holes in the base of each éclair. Pipe the cream into the éclairs, making sure that they are well filled.
To make the glaze, mix the sugar and cocoa powder with 2-2½ tbsp water to make a thick icing that will stay on top of the éclairs without running off.
Dip the top of each éclair into the fondant, or put it into a clean piping bag with a wide nozzle and pipe stripes down the middle. Remove excess fondant, or any drips, with a clean finger. Chill until set, and eat them the day they’re made.