Before our present obsession with chocolate mousse, lovers of French cooking were filling individual porcelain pots with intensely rich, dense chocolate custard. Although this marvelous dessert looks very sophisticated, there is no great secret to success. Just start with the right kind of small, heatproof cups and a good-quality French, Swiss or Belgian bittersweet chocolate such as Callebaut, Valrhona, Tobler or Lindt. Mix the ingredients following the recipe instructions precisely, then strain the mixture to rid it of any lumps. Baking the filled pots in a water bath provides the gentle, moist heat the custard needs to thicken properly.