Heat stock to a simmer and reduce by half.
Add remaining ingredients, except vinegar, liqueur, butter, and seasoning, to the reduced stock and heat to simmer. Cook until desired flavor and consistency are achieved.
Add vinegar, liqueur, and season with salt and pepper, to taste. Skim off any impurities, and adjust consistency as needed.
Strain through a fine sieve into a clean saucepan. Add the butter, stirring constantly, not letting the sauce come to a boil, immediately before serving.