Preheat an ableskiver pan over low heat.

Sift together flour, sugar, baking soda, cardamom, and salt in a bowl.

Create a well in the center of the dry ingredients.

In a separate bowl, whisk together sour cream, milk, and egg yolks.

Pour the wet ingredients into the well of the dry ingredients and stir until well blended.

In a clean bowl, beat egg whites until stiff, rounded peaks form.

Gently fold the beaten egg whites into the batter.

Test the ableskiver pan by dropping a few drops of cool water onto it; if the drops dance around in small beads, the temperature is right.

Grease the wells of the ableskiver pan well with butter (about 1/2 teaspoon per well).

Pour batter into each well, filling about one-half full.

Cook ableskivers, turning frequently with a fork, until golden brown on all sides.

Ensure not to pierce the ableskivers while turning.

Ableskivers are done when a wooden pick inserted in the center comes out clean.

Serve immediately, sprinkled with confectioners' sugar.

If desired, serve with a tart jam.

For apple pancake balls: Rinse, pare, and dice 2 medium-sized apples.

Sprinkle about 1 teaspoon of the diced apples over the batter in each well before cooking.