Put the butter in a saucepan with 225ml water. Weigh the flour in a large bowl and set aside. Bring the butter and water to a fast boil with a pinch of salt, simmer until the butter has melted, then tip in the flour and beat like mad with a wooden spoon until the mixture is pulling away from the sides of the pan and is lump-free. Tip into the bowl and spread the thick paste up the sides a little to help it cool quickly, then leave for 10 mins.
Heat oven to 200C/180C fan/gas 6 and cut 2 pieces of baking parchment to fit 2 baking sheets. Using a ruler and a pen, mark 12 lines about 9cm long over each piece, leaving plenty of space between the lines. Flip the parchment over. Fit a piping bag with a large round piping nozzle, about 1.5cm wide. When the flour paste has cooled but is not cold, start adding the egg, bit by bit, beating well between each addition until you have a smooth batter, which will reluctantly drop off the end of your spoon. You may not need to use all the egg, so add it slowly. Transfer to your piping bag and use a little of the mixture to stick the parchment to your trays.
Pipe the choux onto the baking sheet, using the lines as a guide. Place the trays in the oven and bake for 30 mins until the choux is puffed and golden. Swap the trays around for the final 5 mins. The choux should be crisp and hollow-sounding when tapped. You may wish to sacrifice 1 eclair to make sure that the inside is completely dried out; if it is still moist, the eclairs will sink when cooling. Once baked, leave to cool completely. Can be made 1 day ahead – store in a sealed container, and reheat in the oven for 5-10 mins to crisp up before filling.
Now make the banana custard. Put the bananas, sugar and lemon juice in a food processor and whizz to a purée. Pour into a saucepan and bubble for a few mins until thickened. Mix the custard powder with 2-3 tbsp of the milk to make a smooth paste, then add the remaining milk, cream and vanilla, and stir well. Pour the custard mixture into the banana purée and continue cooking, whisking, until very thick and smooth. Pour into a bowl, cover with cling film and chill for 2 hrs, or until cold.
Pick out 24 nice banana chips and place on a tray lined with parchment. Spray with edible gold spray or cover with a little gold leaf, if you like. When you’re ready to assemble the eclairs, split each one along one side. Transfer the banana custard to a disposable piping bag and snip off the corner. Fill the eclairs generously with the custard, lining up on a tray as you go. Melt the chocolate in a bowl in the microwave, stirring every 20 secs or so to prevent it from burning. When the eclairs are filled, carefully dip the top of each one into the chocolate, letting any excess drip back into the bowl. Place a gold banana chip on each eclair and set back on the tray for 30 mins or so to set. Serve within 3 hrs.