Separate the egg whites and yolks in separate bowls.
Whisk the sugar and egg whites fluffy and stiff. The bowl must be completely clean and dry. A tiny bit of water will make the eggs whites impossible to whisk stiff. Set aside.
Mix the egg yolks, all-purpose flour, natron (baking soda), salt and vanilla sugar together in a separate bowl.
Use a hand mixer to mix the ingredients while gradually adding the buttermilk. Keep mixing until the batter is uniform.
Melt the butter and let it cool off a bit. Slowly add the cooled butter to the buttermilk mixture while whisking.
Use a wooden spoon or similar to slowly mix the stiff eggs whites in the batter.
The Cooking of Aebleskiver
You need a special Aebleskive pan for frying. You can get one online or maybe at your local kitchen store.
Heat up the Aebleskive pan at medium heat and add a small piece of butter in each hole. Fill the holes 3/4 with the Aebleskive batter.
When the batter starts to get firm and you can turn them over without cracking, then turn the Aebleskiver 90 degrees (using a wooden stick or similar) and let the batter flow into the pan.
At this point there is a hole in the side of the Aebleskive. Pour a little extra batter into the hole and turn the Aebleskive another 90 degrees so that the hole gets closed.
When the Aebleskiver have a solid surface turn them regularly so they get an even and light-brown crust.
In step 4 (in cooking of Æbleskiver), try adding different fillings inside the Aebleskiver. I can recommend filling with chocolate chips, strawberry jam or small apple pieces. Serve these Danish aebleskiver together with some jam and icing sugar.