Chop the chocolate into fine pieces and place in a medium bowl; set aside.
Heat the cream in a small saucepan over medium heat until it gently boils. Immediately pour the hot cream over the chopped chocolate.
Let sit for 30 seconds to allow the chocolate to melt, then blend with an immersion blender or whisk until smooth.
Mix in the softened butter.
Transfer the mixture into a large ziplock bag and let it sit at room temperature for about 4 hours, or until thickened.
Line a baking sheet with parchment paper. Snip a corner off the ziplock bag and pipe out small mounds of ganache.
Refrigerate the mounds for 10-15 minutes to firm up.
Sift cocoa powder into a shallow dish. Roll each mound in cocoa to coat.
Store truffles in an airtight container in the refrigerator for up to 2 weeks. Let them come to room temperature before serving.