Chop the chocolate and place it in a large container.
In a saucepan, heat the cream over low heat. Stop just before boiling. Pour the cream over the chopped chocolate. Add the vanilla and diced butter. Let sit for 30 seconds.
Mix with a spatula/maryse starting from the center to create a smooth ganache.
Cover the container on contact to avoid any condensation. Refrigerate for at least 3 hours.
With a tablespoon, scoop out a little ganache and place it as is on a baking sheet covered with parchment paper.
Prepare a plate with bitter cocoa. It is also possible to prepare other coatings (praline, grated coconut, chocolate vermicelli, etc.).
Take each piece of ganache and roll it quickly between your fingers to obtain a nice round ball (my truffles are about 3cm in diameter - you can make them smaller).
Place the formed ball in the bitter cocoa and roll it to coat it well (do the same with the other decorations if desired). Tap off excess cocoa.
Place the finished chocolate truffles on a plate or directly in an airtight container. Refrigerate until ready to eat (take out 15 minutes before so they regain all their flavor).
Notes
These chocolate truffles can be stored in the refrigerator for up to 10 days, in an airtight container.