01 - Place the chopped chocolate in a medium heatproof bowl.
02 - In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer (do not boil).
03 - Pour the hot cream over the chocolate. Let sit for 2 minutes, then add the butter and stir gently until the mixture is smooth and glossy.
04 - Cover the bowl with plastic wrap and refrigerate for at least 2 hours, or until the ganache is firm.
05 - Using a teaspoon or melon baller, scoop out small portions of the ganache and roll them quickly between your palms to form balls (about 1 inch in diameter).
06 - Place the sifted cocoa powder in a shallow bowl. Roll each truffle in the cocoa until well coated.
07 - Store the truffles in an airtight container in the refrigerator. Bring to room temperature before serving.