In a bowl, combine flour, baking powder, sugar, baking soda and salt. Add in egg yolks, melted butter and buttermilk and beat until smooth.
In a separate bowl, beat or whisk the egg whites until they form a stiff peak (it is best to use cold eggs), and then fold the egg whites back into the flour mixture.
Heat Æbleskiver pan until medium hot and put a pat of butter into each well. Put about two tablespoons of batter into each well.
When the bottom is browned, turn the ball of dough so the other side can get browned. Be patient — if you try to turn it too early, it will result in a doughy mess. The browner it gets, the easier it will be to turn. People used to use knitting needles to turn the dough… I used a wooden skewer.
When both sides are browned, remove the Æbleskiver to a plate. They are best served warm, dipped in jam or sprinkled with powdered sugar. There are many variations also — some people stick a piece of apple in the middle when it is half-way done cooking, others use applesauce, Nutella, banana — really, anything you like!