How to make the perfect chocolate truffle

680g bittersweet chocolate, finely chopped
226g milk chocolate finely chopped
1 1/3 cups heavy cream
1 vanilla bean, scraped
11 tablespoons unsalted butter, room temperature
2 cups cocoa powder
(mixtures makes 117 truffles)

Directions
Line a 24cm rimmed baking sheet with plastic wrap; set aside.
In a heatproof bowl, set over (but not touching) simmering water, melt 230g bittersweet chocolate and milk chocolate together.
Place the cream, vanilla bean and seeds in a small saucepan over medium heat; heat until cream just comes to a boil.  Remove from heat.  Let cream mixture cool; remove vanilla bean and discard.
Remove chocolate mixture from heat and add to cream mixture.  Using a spatula, mix together, starting from centre of the saucepan and working your way to the outer edges until mixture is emulsified.  Add butter and mix until well combined.  Pour mixture into prepared baking sheet, evenly spreading mixture with a spatula.  Cover with plastic wrap, pressing down gently on chocolate mixture to prevent a skin from forming.  Refrigerate for at least 2 hours and up to overnight.
Line the back of another 24cm baking sheet with parchment paper.  Uncover chocolate and invert onto parchment paper; remove plastic wrap.  Using a pizza cutter or sharp knive, cut chocolate into 3cm squares.
Line a baking sheet with parchment paper; set aside.  Roll each square of chocolate between your hands to form a smooth ball.  Transfer chocolate balls to baking sheet; refrigerate for 1 hour.
Melt remaining 450g bittersweet chocolate in heatproof bowl,set over (but not touching) simmering water.  Place cocoa powder in a shallow dish.  Place some of the melted chocolate in the palm of your hand and roll chocolate ball in the melted chocolate to coat, then roll in cocoa powder.  Repeat process with remaining chocolate balls.  Transfer truffles to an airtight container.  Bring truffles to room temperature just before serving.