In a bowl big enough to hold all the above ingredients with room to whisk, mix the flour, baking powder, salt and sugar. In a separate smaller bowl, whisk the egg yolks with the milk and 4 tablespoons of the melted butter. Add this mixture into the flour mixture until blended together. Don't worry if the batter looks lumpy.
Using a hand held electric mixer, beat the egg whites on the highest speed until they become stiff but you do not want dry peaks to form. This should take about 3 minutes. Then fold the egg whites back into the flour mixture.
Getting the Pan Ready for cooking
Put the pan over medium heat, add 1/4 teaspoon of butter to each well in the pan and heat until the butter begins to bubble. Add 1 tablespoon of batter to each well of the pan, then 1 teaspoon of filling and top with 1 tablespoon more of batter.
You cook until the bottoms of the round pancakes are golden brown and a little crispy. This should take about 4 minutes. Using a coupe of wooden skewers, flip the pancakes over and cook until the other side is golden brown.
Remove the 7 ebelskiver and repeat with the remaining batter until all the batter is cooked. Serve with maple syrup and powdered sugar.
If you don't want to add filling, that's fine. Just serve them with jam or syrup on the side.