Make sure your milk is no warmer than 43ºC (110F).

Add the brown sugar to the milk, and mix to combine.

Add the yeast to the milk, and stir it through.

Let the milk rest until bubbles appear on the surface of the milk. You're good to go when you see some bubbles appear every second or so.

Put the flour in a mixing bowl, and create a well in the middle.

Sprinkle the salt on the outer edge of the flour.

Add the egg yolks and butter to the well.

Pour the milk, sugar, and yeast mixture into the well.

With a wooden spoon, mix everything together until there are no more lumps.

Cover the bowl, and let the batter stand in a warm spot for 1 hour.

Preheat your oil to 180ºC (350F)

In a shallow dish, combine the cinnamon and sugar, and stir it thoroughly.

Peel and core the apples.

Slice the apples into medium slices

Dip the apple slices in the cinnamon-sugar mixture.

Dredge the apple slice through the batter, making sure it's covered. If the batter is too thick for this, you can thin it out with some milk.

Fry the appelflappen for 3 minutes, flipping them over halfway through. Don't overcrowd the oil, only do a few at a time.

Serve warm, covered in powdered sugar, or more cinnamon sugar.