Preheat the oven to 200°C and line a tray with parchment.
In a saucepan, combine 125ml water with the butter over low heat.
Bring the mixture to a boil and immediately stir in the flour and sugar to form a paste.
Remove from heat, cool slightly, and gradually beat in the eggs until glossy.
Spoon 18 mounds of paste onto the tray, spacing them well apart.
Bake for 25 to 30 minutes until well risen and golden.
Pierce a 1cm hole in the base of each bun and cool on a wire rack.
Melt white chocolate with 150ml cream over low heat until smooth.
Cool the white chocolate sauce for 30 minutes until it whitens.
Melt dark chocolate with 100ml cream in the same manner and set aside.
Whip 350ml cream with sugar and vanilla and pipe into the cooled buns.
Thin the dark sauce with hot water, pour over buns, and drizzle with white chocolate.