Carefully separate the egg whites from the egg yolks of the 4 eggs. Place the egg whites in a clean, dry bowl.
Using a whisk or electric mixer, beat the egg whites until they become fluffy like clouds and form stiff peaks when you lift the whisk. This means they should hold their shape.
In a separate bowl, combine the egg yolks with the sugar. Mix them well until the mixture is light and well blended.
Add the milk and the vanilla sugar to the egg yolk mixture. Mix again until everything is smoothly incorporated.
Gradually add the flour to the egg yolk mixture, mixing until the batter becomes thicker than a pancake mix. It should be a smooth, pourable but substantial consistency.
Now, gently fold the egg yolk mixture into the whipped egg whites. Use a spatula and combine them carefully, using an upward motion, to ensure the mixture remains fluffy and doesn't lose its airy texture.
Finally, gently fold in the prepared cherries into the cake batter. Be careful not to overmix, as you want to keep that fluffy texture.