Heat heavy cream in a small saucepan over medium heat until it just begins to simmer and small bubbles form around the edges. Remove from heat immediately.
Pour the hot cream over the finely chopped chocolate and butter in a heatproof bowl. Let it sit undisturbed for 2 minutes, then stir slowly from the center outward until completely smooth and glossy. Stir in the vanilla extract.
Cover the bowl with plastic wrap and refrigerate for at least 2 hours until the ganache is firm enough to scoop and hold its shape.
Using a teaspoon or small melon baller, scoop portions of the ganache and quickly roll into 1-inch balls between your palms. Place on a parchment-lined tray. If the ganache becomes too soft, return it to the refrigerator for 10 minutes.
Mix cocoa powder and powdered erythritol together on a shallow plate. Roll each truffle in the mixture until evenly coated on all sides.
Place the finished truffles back on the parchment-lined tray and refrigerate for at least 30 minutes before serving. Store in an airtight container in the refrigerator for up to 2 weeks.
