Finely chop the chocolate and place it in a medium bowl. Set aside.
Place the cream in a small saucepan and bring to a gentle boil over medium heat (or warm the cream in the microwave). As soon as the cream begins to boil, remove it from the heat and pour it over the chopped chocolate in the bowl.
Let the mixture sit for 30 seconds, then whisk (or blend with an immersion blender) until smooth. Stir in the butter until fully incorporated.
Transfer the ganache to a large zip-top bag and seal it. This makes it easy to pipe once the ganache has thickened. Let it rest at room temperature for 4 to 6 hours, or the fridge for 1 to 2 hours until thick enough to hold its shape.
Line a baking sheet with parchment paper. Clip a corner of the bag off and pipe out small mounds of the chocolate ganache mixture (a little bigger than a teaspoon). If the ganache warms up in your hands while piping and softens too much, place the bag in the fridge for a few minutes.
Place the mounds of piped chocolate in the refrigerator for 15 to 30 minutes to firm them up.
Sift the cocoa powder into a pie plate or shallow dish. Roll each mound of chocolate into a ball then roll them in the cocoa powder to coat.
Notes
Use good-quality chocolate: Choose a dark chocolate with 55% to 70% cacao for the best flavor and a smooth, creamy ganache. Chocolate bars melt more evenly than most chocolate chips.European-style butter: European-style butter, such as Kerrygold or Plugrá, has a higher butterfat content, giving the ganache an extra silky texture.Let the ganache thicken naturally: Allowing the ganache to rest at room temperature creates a smooth texture that's easy to pipe. It's ready when it holds its shape after being piped into a mound.Storage: Store the truffles in an airtight container in the refrigerator for up to 2 weeks. Let them sit at room temperature for 15 to 20 minutes before serving for the best flavor and texture.Freezing: Freeze in an airtight container for up to 3 months. Thaw overnight in the refrigerator, then let the truffles come to room temperature before serving.