Prepare the filling first. For ganache, heat the heavy cream until steaming but not boiling, pour it over the chopped chocolate, and let it stand for 2 minutes. Stir until smooth, then mix in the optional butter, salt, and vanilla; cool until thick but still pipeable.

Make sure the mold, bowls, spatula, knife, and piping bag are completely clean and dry. Finely chop the couverture chocolate and reserve about one-third for tempering.

Melt about two-thirds of the chopped chocolate gently over a double boiler or in short microwave intervals, stirring frequently. Do not allow steam or water to contact the chocolate.

Temper the chocolate by melting dark chocolate to 113-122°F (45-50°C), milk chocolate to 105-113°F (40-45°C), or white chocolate to 104-113°F (40-45°C). Stir in the reserved unmelted chocolate, then cool dark chocolate to 88-90°F (31-32°C), milk chocolate to 86-88°F (30-31°C), or white chocolate to 82-84°F (28-29°C).

Spoon or pipe tempered chocolate into the mold, then turn it upside down over the bowl to drain away excess chocolate. Scrape the mold clean, leaving shells about 1/8 inch (3 mm) thick, and let them partially set for several minutes.

Pipe or spoon the cooled filling into each shell, stopping about 1/8 inch (3 mm) below the rim. Add optional nuts, coconut, dried fruit, or cookie crumbs without overfilling the cavities.

Cover the filling with the remaining tempered chocolate to create a solid chocolate foot. Scrape the mold flat and tap it gently on the counter to remove air bubbles.

Let the chocolates set for 30-60 minutes at cool room temperature, or refrigerate them for 15-20 minutes. Unmold only when the chocolate is firm and releases easily, then store the finished pieces in an airtight container.