Step 1In a medium bowl, whisk together the flour, baking soda and sugar. Slowly drizzle the buttermilk into the mixture while whisking. Add the egg yolks and mix well to combine and form a batter. Whisk in the butter and vanilla extract, if using.
Step 2In the bowl of a stand mixer, or in a large bowl using a hand mixer, whisk the egg whites until stiff peaks form. Gently fold the whites into the batter.
Step 3Set the batter aside at room temperature for 15 minutes to rest.
Step 4Heat the aebleskiver pan over medium heat, then brush each mold lightly with melted butter. When the butter just begins to brown, pour the batter so each mold is filled almost to the rim. (Start with the middle mold as it receives the least amount of heat and remember the sequence in which you fill each mold in the pan, as the aebleskivers will need to be turned in that order.)
Step 5When a solid crust has formed on the bottom of each mold, reduce the heat slightly and use a skewer to loosen and turn each aebleskiver by one-third. As the crust continues to form, turn each by one-third again. Finally, turn each mold again so the entire crust is formed. Reduce the heat to low and cook until the interior of each aebleskiver is cooked through. A toothpick or skewer inserted should come out clean.
Step 6Use a skewer to remove the aebleskivers from the pan. Plate in arrangements of three; sprinkle with powdered sugar and top with tart berry jam. Serve immediately.