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