Chocolate shell
A shell is the thin chocolate casing of a moulded bonbon, cast in a mould and then filled. Shell work is a whole discipline of its own, distinct from truffles, where the filling has no rigid casing.
The mould is filled with tempered chocolate, tapped to release air, then inverted so the excess runs out and leaves a layer on the walls. Thickness is controlled by the temperature of the chocolate and the draining time: too thin and the shell breaks on demoulding, too thick and it overwhelms the filling.
Once set, the shell is filled, leaving room for the base, and the filling is left to stabilise so the bonbon does not separate. The shell is then capped with the same tempered chocolate.
Properly tempered chocolate contracts as it sets and pulls away from the mould walls on its own — which is why bonbons drop out of a polycarbonate mould without force. If they stick, tempering is almost always the reason.