Place the chopped dark chocolate in a medium heatproof bowl and set aside.

Heat the heavy cream in a small saucepan over medium heat until it just begins to simmer, with small bubbles forming around the edges. Do not let it boil.

Pour the hot cream over the chopped chocolate and let it sit undisturbed for 2 minutes to allow the chocolate to soften.

Whisk the mixture gently from the center outward until completely smooth and glossy, then stir in the softened butter and vanilla extract until fully incorporated.

Cover the bowl with plastic wrap and refrigerate for 2-3 hours until the ganache is firm enough to scoop but not rock-hard.

Line a baking sheet with parchment paper. Using a small cookie scoop or teaspoon, scoop out portions of ganache and quickly roll each between your palms into a smooth ball, working rapidly to prevent melting.

Place the cocoa powder in a shallow bowl. Drop each truffle into the cocoa and roll gently with a fork to coat evenly, then transfer to the prepared baking sheet.

Refrigerate the coated truffles for at least 30 minutes to set completely before serving or packaging.