If you google "liquore al cioccolato", you will find dozens of recipes for making it yourself; most of the the recipes are in Italian, unfortunately.
Since you were served it in Sicily, I assume - perhaps wrongly - that it was the liquore al cioccolato of the Antica Dolceria Bonajuto in Modica. The Antica Dolceria Bonajuto has a Web site, and it claims to deliver all over the world.
If you scroll down, you will see Xocolic liquore al cioccolato, made without milk or thickening agents. There are two versions, one plain chocolate, the other chocolate flavored with chili peppers ("peperoncini"); I would think you want the plain one...
Thank you for the information Eloise. I had a different brand that was purchased at across from the Duomo in Polermo!! I may try to order some of that brand though!!