Place the chopped dark chocolate in a medium heatproof bowl and set aside. Heat the heavy cream in a small saucepan over medium heat until it just begins to simmer with small bubbles around the edges, about 3-4 minutes.

Pour the hot cream over the chopped chocolate and let it sit undisturbed for 2 minutes to soften the chocolate. Add the softened butter and salt, then stir gently from the center outward with a spatula until the mixture is completely smooth and glossy.

Cover the bowl with plastic wrap and refrigerate for 2-3 hours, or until the ganache is firm enough to scoop but not rock-hard. It should hold its shape when you press a spoon into it.

Line a baking sheet with parchment paper. Using a small cookie scoop or melon baller (about 1 tablespoon), scoop portions of ganache and drop them onto the prepared sheet. Refrigerate for 15 minutes to firm up slightly.

Place the cocoa powder in a shallow bowl. Dust your hands lightly with cocoa powder, then quickly roll each ganache portion between your palms into a smooth ball, working rapidly to prevent melting.

Immediately roll each truffle in the cocoa powder to coat completely, then return to the parchment-lined sheet. Refrigerate for at least 30 minutes before serving to allow them to firm up fully.

Before serving, let the truffles sit at room temperature for 5-10 minutes so the ganache softens to a creamy, melt-in-your-mouth texture. Store leftovers in an airtight container in the refrigerator between layers of parchment paper.