In a bowl, whisk together flour, sugar, baking powder, baking soda, and salt.
In another bowl, whisk together buttermilk, egg yolks, and melted butter.
Combine the wet and dry ingredients until just mixed.
In a separate bowl, beat egg whites until stiff peaks form. Gently fold into the batter.
Heat the aebleskiver pan over medium heat and lightly grease each well.
Fill each well halfway with batter. Cook until edges start to set, then use a skewer to turn each pancake ball 90 degrees. Continue cooking, turning to form a ball, until golden brown and cooked through.
Serve warm with powdered sugar, jam, or syrup.