They're called "æbleskiver", which means "appleslices" because we used to make them by battering slices of apple in a dough that would raise a lot. They're made in a specific pan, which you can get shipped to you from the town Elk Horn apparently, which was founded by Danish immigrants. They eat it as breakfast, which strikes me as odd, but Americans eat pancakes for breakfast so it makes some sense. They're traditionally served with jam and powdered sugar.