In a small bowl whisk together the egg yolks, milk and 2 TBSP of the melted butter. Add the yolk mixture to the flour mixture and mix with a spoon, it will be lumpy.
Beat the egg whites on high speed till they form stiff peaks. Pour about a third of the whites into the yolk/flour mixture and lightly fold it in with a spatula. Then add the rest of the whites and fold it in till no white streaks remain.
Brush the ebelskiver pan with melted butter and heat till the butter bubbles. Add about 3 tablespoons of the batter to each hole, add filling* in the middle, and cook till the bottom is crispy, about 3 -5 minutes. Flip to the other side. Cook on the other side till lightly browned, about another 3 minutes. Wooden skewers are a great tool for flipping them. Dust with confectioner’s sugar to serve.
*Good combinations: little chunk of chocolate, ½ tsp jam + little chunk of chocolate, ½ tsp lemon curd + little chunk of chocolate, nutella + little chunk of chocolate, cooked apples or pears + little chunk of chocolate