Preheat oven to 220°C and line a tray with eight 5cm circles and eight 2.5cm circles.
Melt butter in 150ml water, bring to a boil, then remove from heat.
Stir in flour until a soft ball forms, then cook over low heat for 5 minutes.
Cool slightly and beat in eggs gradually to form a shiny paste.
Pipe the paste into the marked circles and smooth the tops with a damp finger.
Bake for 10 minutes at 220°C, then reduce to 190°C for 15 minutes.
Pierce each bun with a skewer and return to the oven for 5 minutes to dry.
Bring milk and vanilla to a boil, then whisk into a mixture of yolks, sugar, and flours.
Return custard to the pan and cook for one minute until thickened.
Cover the custard with film and chill in the fridge.
Boil 150ml cream and stir in chocolate to make a ganache.
Fill the cooled buns with chilled custard using a piping nozzle.
Dip buns into ganache and stack small buns on top of larger ones.
Whip remaining cream to soft peaks and pipe collars around the bun joins.