Advocaat is a liqueur made from a rich and creamy blend of egg yolks, aromatic spirits, sugar and brandy with a hint of vanilla. It has a smooth and full-bodied taste.
There are two varieties of Advocaat. The "thick" Advocaat which is sold mostly on the Dutch market and eaten with a spoon, and the more liquid "export" version.
Thick Advocaat also contains egg white, and is used as an ingredient for all kinds of desserts, for instance with ice-creams and pastries. It is also served as an aperitif or digestif. The traditional way to serve it is in a wide glass with whipped cream and cocoa powder sprinkled on top.
In the export variety only the pure egg yolks are used, making it particularly suited for cocktails and long-drinks. The best known cocktail using Advocaat is probably the Snowball.
Source: Bols Royal Distilleries
In addition to Bols Advocaat, De Kuyper also make it under the name Warninks Advocaat.
You can make it yourself.