In a medium bowl, beat the egg whites until stiff. In another bowl, whisk together the egg yolks, buttermilk and melted butter.
In a large bowl, whisk together the flour, sugar, baking soda, cardamom and salt. Stir in the buttermilk mixture, and fold in the egg whites.
Heat a cast iron Æbleskiver pan over medium-high heat and brush generously with oil or butter—enough that there’s about half a teaspoon in the bottom of each divot. Spoon a spoonful of batter into each and cook, rotating with a fork or bamboo skewer as the bottom turns golden. Serve immediately or keep warm in a baking dish in a 250?F oven. Makes about 2 dozen Æbleskiver.