For the apple cinnamon muffins, preheat the oven to 180 degrees (upper-lower heat) and place the paper cups in the muffin tin.
Beat the eggs, butter, sugar and vanilla sugar with a mixer until fluffy. Combine flour, baking powder and salt in a bowl.
Add first one half to the egg-butter-sugar mixture and mix well, then the second. Add yogurt and cinnamon and mix again until homogeneous.
Core and peel the apples, cut them into cubes, add them and mix everything well with a tablespoon. Now fill the paper cups with the batter to just below the rim (it’s best to use a tablespoon).
For the cinnamon crust, mix all the dry ingredients in a bowl, then add the butter and rub everything between your fingers until everything is well mixed.
Spread the sticky mixture over the apple cinnamon muffins and place them in the oven for about 20-25 minutes.