Regan's Orange Bitters No. 6 is, predictably, the result of six full tries to get the recipe just right. Made by Gaz Regan in the comforts of his own kitchen, and later perfected by Mark Brown and The Sazerac Company out of New Orleans, this citrus bitters is ideal for any cocktail that needs that familiar citrus kick. The secret to the recipe involves gentian, cinchona, and quassia. Further details here would be getting into trade secret territory.