In medium bowl, combine egg yolks, buttermilk, vanilla, and melted butter, whisking to combine.

In large bowl, combine flour, baking powder, salt, and sugar, mixing to combine.

In separate bowl, beat egg whites until medium peaks form.

Add egg yolk mixture to flour mixture, stirring until just combined. It will be lump and that's ok!

Fold in the whipped egg whites.

Heat aebleskiver pan over medium heat.

In small bowl, combine remaining melted butter and vegetable oil.

Grease each aebleskiver hole with butter/oil mixture, then add batter to each hole, filling just half way.

Working quickly, drop 1 teaspoon of Nutella into each batter-filled hole and then top with more batter, until hole is almost completely filled.

Cook for about 2 minutes, until the edges look like they are beginning to set, and using a skewer or aebleskiver turner, turn about ⅓ of the way. Cook another 1-2 minutes, and then turn one last time, completely over to seal the aebleskiver.

Cook another minute, and then remove from pan using skewer, fork, or aebleskiver tool.

Continue until all batter is used.

Sprinkle with powdered sugar or top with maple syrup.

Enjoy!