In a saucepan, add milk, water, butter, sugar and salt. Bring to boil.
Once boiled, reduce the heat to medium. Add in the flour and stir vigorously until a dough lump is formed. Cook until a thin layer forms at the base of the saucepan.
Transfer the dough lump into a mixer and cool the dough by whisking it continuously for at least 2 minutes.
Once the temperature is slightly lowered, add the eggs one at a time, (otherwise, the eggs would cook if the temperature is still high) whisk until all the eggs are absorbed into the dough and the dough becomes glossy. After that, transfer it into a piping bag.
Pipe it into a semi-sphere silicone mould. After filling the silicone mould, lightly flatten the tip of the dough with a little bit of water and freeze the dough.
Once frozen, remove the buns from the silicone mould and place them on a floured baking tray. Make sure there is a 5 cm gap in between each choux buns.
Half-way through thawing, put a choux cap on top of each bun.
Preheat the oven at 190°C and start baking the choux buns at the lowest fan for 10 minutes.
After 10 minutes, lower the temperature to 160°C and bake for another 10 minutes.
Remove the buns from the baking tray and rest them on a cooling rack.
Once cooled, pipe the filling by lightly poke a 5 mm hole on the base of the choux bun to inject the durian custard filling.