Prepare all ingredients. Remove the butter from the refrigerator ahead of time and cut it into softened pieces. Beat the softened butter and sugar with a mixer. If you don't have a mixer, a fork will do; mix the ingredients by hand. Add the eggs and mix thoroughly until smooth.
In a separate bowl, combine the dry ingredients: oats, flour, baking powder, salt, cinnamon, and vanilla sugar. Sift the flour with the baking powder first. Stir the mixture and pour it in several batches into the bowl with the liquid mixture of butter, eggs, and sugar. Mix all ingredients thoroughly.
Wash the apples, core them, peel them, and cut them into very small cubes. Add the apples to the dough and mix everything together (you can use your hand). Refrigerate the dough for 15–20 minutes. The dough shouldn't be too soft, otherwise it will be difficult to shape the cookies.
Line a baking sheet with parchment paper or a silicone baking mat. Use a teaspoon to scoop out cookies, smooth them out, and place them on the baking sheet at slightly spaced intervals. It's easiest to shape the cookies with wet hands. Press down with a spoon to shape them. You can decorate the cookies with crushed nuts or coconut flakes.
Preheat the oven to 175 degrees Celsius (350 degrees Fahrenheit). Bake the cookies for 15–20 minutes, until golden brown. Cooking time will vary depending on the size and height of the cookies. It's important to remove them promptly to avoid overcooking them. Place the finished cookies on a flat surface to cool completely, ensuring they are level. They will feel springy immediately after baking, but will harden a little later.