First, warm up the milk and keep it hot on the stove. Beat the egg yolks with the sugar in a mulled wine mug, punch glass or similar until frothy. Add rum and fill up with hot (not boiling!) Milk while stirring and serve hot.
Tip: If you like, you can top it off with a cherry on top and chocolate sprinkles.
Note: There are also recipes in which water is used instead of milk - but I definitely like the milk variant better. The amount of rum can of course be varied up and down as desired - 8 cl per serving should not, however, be exceeded.