I used chocolate bitters. Next time I will add a drop of saline, go all out on expressing with orange peel, plus put a piece of Lindt dark chocolate orange on the side.
I used older, sweeter version of rabarbaro, giffard creme de cacao White and bitter truth chocolate bitters. The base was ocho blanco. Nice coctail with tequila to the fore, and medium chocolate/cacao biterness but something (dash or two of absinthe) is missing.