
Walnut liqueur is a sweet liqueur flavored with walnut, most often from unripe green walnuts macerated in spirit.
Another walnut liqueur
Use at the same measure, but remember that strength and sweetness vary widely.
Use hazelnut liqueur in the same amount with 1–2 dashes of black walnut bitters to put back some of the green bitterness. Expect a slightly weaker, rounder cocktail.
It is the unripe green fruit, husks and all, that gives the near-black color.