Temper the chocolate. Melt two thirds of the dark chocolate to 45–50 °C over a water bath or in 20-second microwave bursts. Off the heat, stir in the remaining third until it melts and the chocolate cools to 31–32 °C. A smear on a knife tip should set glossy within 5 minutes.

Make the shells. Fill each mould cavity to the top and tap the mould to release air bubbles. Turn it upside down over the bowl and let the excess run out, then scrape the surface clean. Let the shells set, cavities down, for 10–15 minutes.

Make the ganache. Bring the cream just to a simmer and pour it over the chopped milk or dark chocolate. Wait 1 minute, then stir from the center until smooth. Stir in the butter and vanilla. Let it cool to about 30 °C.

Fill. Pipe the ganache into the shells, stopping 2 mm below the rim. Let it firm up at cool room temperature for at least 1 hour.

Cap. Re-warm the leftover tempered chocolate to its working temperature, spread it over the mould and scrape off the excess. Let it set for 20 minutes at room temperature, then 10 minutes in the fridge.

Unmold. Turn the mould over and tap it firmly on the counter. Well-tempered bonbons drop out on their own; if some stick, chill 5 more minutes and try again.