- Place a 10-øre coin in an empty coffee mug.
- Pour black coffee into the mug until you can no longer see the coin.
- Pour akvavit into the mug until you can see the coin again.
Nota bene: Do not drink the 10-øre coin.
Addendum: Thanks to arj for bringing to my attention the node Karsk, which describes essentially the same drink.
Another addendum: liveforever cautions us against taking the name 'Viking coffee' literally, and points out another term for the beverage in question:
"Viking coffee", my hairy arse. Distilling postdates the viking era. What you're describing is what in Danish is known as "en (lille) sort" = "a (small) black".