2 cups cocoa powder (not Dutch processed), preferably Valrhona, for rolling
Line a 13-by-9 1/2-inch rimmed baking sheet with plastic wrap; set aside.
In a heatproof bowl, set over (but not touching) simmering water, melt 8 ounces bittersweet chocolate and milk chocolate together until it reaches 120 degrees.
Place 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 until reaches 120 degrees; remove vanilla bean and discard.
Remove chocolate mixture from heat and add to cream mixture. Using a heatproof spatula, mix together, starting from the center of the saucepan and working your way to the outer edges until mixture is emulsified. Add butter and mix with an immersion blender until well combined. Pour mixture into prepared baking sheet, evenly spreading mixture with an offset spatula; cover with plastic wrap, pressing down gently on chocolate mixture to prevent a skin from forming. Refrigerate at least 2 hours, and up to overnight.
Line the back of another 13-by-9 1/2-inch baking sheet with parchment paper. Uncover chocolate and invert onto parchment paper; remove second piece of plastic wrap. Using a pizza cutter or sharp knife, and a steel ruler as a guide, cut chocolate into 1-inch 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 16 ounces bittersweet chocolate in a 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 a 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, stacking truffles no more than 2 to 3 inches high, and refrigerate up to 2 weeks. Bring truffles to room temperature just before serving.