Place sugar and chocolate into the Thermomix bowl and mill 20 sec/speed 9. Scrape down sides of the bowl.
Add remaining ingredients, except butter and cook 10 min/90°C/speed 4.
Add butter and blend 10 sec/speed 6.
Pour into a glass/ceramic dish and cover with plastic wrap or baking paper, pushing the wrap down onto the top of the custard to form a cartouche.
Cool completely in the fridge before piping into the centre of profiteroles. Alternatively it does work as a chocolate pudding whip. Cue the cream!
If making white chocolate pastry cream you can omit the sugar as generally white chocolate is a lot sweeter than dark chocolate.