Add Angostura and orange bitters.
Angostura bitters has been made to essentially the same formula since 1824. Two dashes is approximately 1ml — a small volume, but one that adds a complex herbal and spice profile (gentian, cinchona bark, various botanicals) that makes the drink multidimensional rather than simply sweet whiskey. Orange bitters adds a brighter citrus counterpoint to the darker, more herbal Angostura.