Line a small tray or plate with baking parchment or a silpat mat. Set aside.

Melt dark chocolate over a double boiler or bain marie.

Meanwhile, combine Guinness and whipping cream in a small saucepan and warm gently over medium-low heat. (Do not boil.)

Gently whisk the warm cream mixture into melted chocolate, followed by the softened butter.

Transfer bowl to the refrigerator and chill, at least one hour or until firm enough to scoop.

Once thoroughly chilled, use a melon baller, small cookie scoop, or tablespoon to scoop the ganache, using your hands to smooth and shape each truffle into a round ball. Roll each ball in cocoa powder and transfer to prepared tray. Chill in the refrigerator for 15-20 minutes.

Remove chilled truffle balls from the refrigerator and roll in a second coating of cocoa powder.

Store truffles in an airtight container in the refrigerator for up to three days. Re-roll in cocoa powder prior to serving. Serve in paper truffle cups.

Store leftover truffles in an airtight container in the refrigerator for up to five days.