Separate the eggs.
Beat the egg whites in a clean bowl until stiff peaks form. Set aside.
Mix the batter.
In a large bowl, whisk together flour, baking powder, baking soda, salt, and sugar. In another bowl, mix egg yolks, milk, melted butter, and vanilla (if using). Combine wet and dry until smooth (don't overmix).
Fold in the egg whites.
Gently fold whipped whites into the batter in two additions. This gives the bleskiver their light, puffy texture.
Preheat your æbleskiver pan over medium-low to medium heat. Brush each well with butter or oil.
Scoop and cook.
Fill each well about ¾ full with batter. Let cook 1-2 minutes until bubbles appear and edges firm. Use a skewer, knitting needle, or fork to gently turn each ball 90°, letting uncooked batter flow down. Turn a few more times until fully round and golden.
Serve warm!
