1. Start by making the custard filling. Heat the milk until almost boiling in a saucepan. Meanwhile, mix together the sugar, egg yolks and vanilla in a bowl, then stir in the flours, a couple of tsp at a time, to a smooth paste.
2. Gradually whisk in the hot milk, pour everything back into the saucepan and cook over a high heat, stirring constantly, for about 5 mins until thick – it will go alarmingly lumpy but don’t worry, just keep stirring it vigorously with a wooden spoon until smooth. Lay a sheet of cling film directly on the custard surface, then cool and chill until you’re ready to fill the buns.
3. To make the choux buns, heat oven to 220C/200C fan/gas 7. Sift the flour with the sugar and a pinch of salt into a small bowl.
4. Put the milk and butter into a medium saucepan with 125ml water and gently heat so the butter melts but the liquid doesn’t bubble. Once the butter has completely melted, increase the heat until the liquid comes to a fast rolling boil. Immediately turn off the heat, tip in the sifted flour and beat vigorously with a wooden spoon until you a have smooth dough that comes away from the sides of the pan. Spread over a large dinner plate to cool to hand temperature.
5. Once the dough mix has cooled, scrape it back into your pan. Using your wooden spoon, beat in each egg, one by one, until you have a smooth, shiny mixture.
6. Cut 2 large sheets of baking parchment. On each one draw 2 sets of ‘track’ lines with a 10cm gap – these will be your guidelines so your eclairs will all be roughly the same size.
7. Use the paper to line 2 large baking sheets – penside down. Spoon your choux mixture into a piping bag with a 1cm wide nozzle, or into a disposable piping bag with a similar-size hole snipped off for piping. Pipe 2 rows of well-spaced, squashed ‘S’ shapes on each sheet between the guidelines (see picture 3). Bake, one tray at a time, on a high-ish shelf for 25 mins, reducing temperature to 200C/180C fan/ gas 6 as soon as they go in the oven.
8. After 25 mins, poke a hole in the end of each bun, or using a small serrated knife, split down the middle and return to the oven, upside-down, to dry out for 5 mins until crisp and golden. Set aside to cool.
9. While the buns are cooling, finish your filling. Whisk the cream until thick, then use your electric whisk to beat the cooled, set custard until just smooth again.
10. Fold in the cream. Spoon your filling into a piping bag – use a small nozzle if you’re filling the buns through the holes you've pierced, or a large nozzle if you’ve split the buns in half.
11. Carefully pipe the custard into each cooled bun – they should feel heavy once full. Arrange the buns on a wire rack and spread each with a little of your chosen icing. If you can’t decide, it’s really easy to split your batch of choux buns, halve icing quantities, and make a selection to keep everyone happy.