A Fiaker is strong black coffee with rum or kirsch, sugar, and a cap of whipped cream — an Einspänner with a spirit in it.
The name
Fiaker is the Viennese horse-drawn cab, and the drivers are Fiaker too. Like the Einspänner — named after a one-horse carriage — the drink belongs to the same working world.
Both were, in origin, coffees a driver could hold in one hand while keeping the other on the reins, with a cream lid keeping them hot.
Making one
A double espresso or a strong long black in a handled glass. Sugar to taste, 20 to 30 ml of rum, stirred in. Thick unsweetened whipped cream spooned on top.
Drink through the cream without stirring.
Do not sweeten the cream
The balance depends on unsweetened cream against sweetened coffee. Sugar in both makes it cloying.
The Viennese family
Einspänner (no spirit), Franziskaner (a Melange with cream), Kapuziner (black with a little cream), Maria Theresia (with orange liqueur).