Place the finely chopped dark chocolate in a heatproof bowl.

In a small saucepan, heat the heavy cream and butter over medium heat until just simmering. Do not boil.

Pour the hot cream mixture over the chopped chocolate. Let it sit for 5 minutes without stirring.

Gently whisk the mixture until smooth and emulsified, forming a ganache. Stir in the vanilla extract.

Cover the bowl with plastic wrap, pressing it directly onto the surface of the ganache to prevent a skin from forming. Chill in the refrigerator for at least 2-3 hours, or until firm enough to scoop.

Once chilled, scoop out portions of the ganache using a small spoon or melon baller. Roll each portion into a ball between your palms. Work quickly as the warmth from your hands will melt the truffles.

Place the cocoa powder (or other desired toppings) in a shallow dish. Gently roll each truffle in the cocoa powder to coat evenly. Place the finished truffles on a parchment-lined baking sheet.

Allow the truffles to set at room temperature for about 30 minutes before serving, or store them in an airtight container in the refrigerator.