Mix all-purpose flour, sugar, ground cardamom, baking soda, and salt in a medium-sized mixing bowl.
Separate egg whites and egg yolk.
In another mixing bowl, beat egg whites until stiff.
Add the egg yolks and buttermilk to the flour mixture and mix well. Add the melted butter and continue mixing until the dough is smooth.
Add the stiff egg whites to the dough and stir carefully.
Heat up the æbleskive pan, and add a bit of butter in each hole.
When the pan is hot, fill up each hole with 2/3 dough. Remember the dough will rise.
After a few minutes, flip the æbleskive on the side and add 1 teaspoon of apple filling. Use a thin metal stick or alternatively a fork. Fill up the hole with more dough and turn the æbleskive all the way around. Let it bake for a few minutes until all sides are golden.
Serve the æbleskiver immediately. The æbleskiver is supposed to be dipped in a berry jam and powdered sugar.