1.In a medium saucepan, melt the butter; add the milk and stir over low heat until mixture reaches body temperature, about 37°C. (If mixture is too hot, it will kill the yeast.) Add the yeast to the milk mixture; stir to dissolve.
2.Add the sugar, salt and cardamom or vanilla. Place the flour in a large bowl; make a well in the centre. Gradually add the milk mixture to the well and whisk flour into the milk mixture until a smooth batter forms. Whisk in the eggs. Place the batter in a warm place and allow to proof about 1 hour or until doubled in size.
3.Heat an aebleskiver pan or heavy-based frying pan over medium. Add a small piece of the extra butter to each hole; fill the holes 3/4-full with batter, about 1 tablespoon in each. When the base of the aebleskiver looks browned and the edges are cooked, about 1 minute, turn them over using two bamboo skewers; then cook until cooked through.
4.Remove from pan with skewers; transfer to serving plate. Serve sprinkled with extra caster sugar and good marmalade to dip in. Repeat with remaining mixture, adding a little butter to pan holes between each batch. Adjust the heat if necessary, so aebleskiver cook evenly.
Add some grated orange or lemon rind to the batter, if desired. A warm place to proof the batter is on the open door of a very slow oven, in a sunny spot or near a heater.Note