1: Cook the mussels in a little boiling salted water for 1 or 2 minutes, just until they open. For a more delicate flavour, you can also cook them like for mussels marinière. Shell the mussels.
2: Preheat the oven to 390°F (200°C). Put each mussel in a little paper petit-four case.
5: Sit a morsel of beurre d'escargot on top of each mussel.
6: Cook in the oven for 5 or 6 minutes at the most. Above all, do not let the butter brown. Serve piping hot.
Remarks
If you don't have any paper cases, you can use half a mussel shell instead, but this is less practical as the capacity is generally a bit small. Another more elegant solution, though rather more complicated, is to make little cases out of puff or flaky pastry (pâte feuilletée).