Step 1In a large bowl, whisk together the warm milk and yeast; set aside until the mixture begins to bubble, about 5 minutes. Whisk in the eggs and sugar until combined, then add the butter, vanilla, lemon juice and zest. Finally, add the flour and whisk until the batter is smooth.
Step 2Cover the bowl loosely with plastic wrap and set aside in a warm place until the batter is at least doubled in size, about 30 minutes. The batter will be thick and slightly sticky.
Step 3Heat 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 4When 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 continue to cook until the interior of each aebleskiver is cooked through. A toothpick or skewer inserted should come out clean.
Step 5Use 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.