Chocolate mint liqueur is similar to normal chocolate liqueur only with spearmint flavor added. This product tends to be cream based (shelf stable cream) like Bailey's but if you can find a straight version that omits the cream go for it. Most recipes that use chocolate mint liqueur already have cream in the recipe.
If you can't find chocolate mint liqueur or don't want to stock it in your bar for the two times per year you need it, you can easily adapt normal chocolate liqueur (whether creme de cacao or Godiva). Just add a few drops of spearmint tincture, a teaspoon of so of mint syrup, or muddle mint leaves in the shaking vessel.