Put 12 ounces of the chocolate in a bowl. Whisk together the cream and corn syrup in a saucepan and bring to a simmer. Pour over the chocolate and whisk until thoroughly combined and smooth. Whisk in butter until smooth and glossy. Add the raspberry liqueur and mix well.
Pour the chocolate into a deep pan and chill. When the chocolate is firm, scoop out small truffle balls, using a melon baller or other small scoop. (Dip scoop in very hot water before each scoop, patting each time onto a towel to avoid getting water into the chocolate mixture.) Chill truffles slightly, and then perfect the circular shape by rolling quickly in palms of your hands. Store the truffles in the refrigerator until ready to finish.
To finish the truffles: Melt the remaining 8 ounces of chocolate in a bowl over a saucepan of simmering water, stirring occasionally. Set the bowl aside and let cool slightly, stirring occasionally, until just cooler than body temperature.
Put enough cocoa powder in a bowl to form a small pile. Wearing rubber gloves and working with 1 truffle at a time, rub some of the melted chocolate all over the truffles and roll quickly in your hands until smooth. Drop the truffles into the cocoa powder, roll to coat completely, and transfer to a platter. Store in a cool, dry place.