Preheat oven to 350°F (180°C) and fold a 7×7 in (15 cm x 15 cm) brownie pan with parchment paper.
Wash, core, and optionally peel the apple. Cut it into chunks.
In a high speed blender, combine the diced apple with egg, Greek yogurt, rolled oats, maple syrup, ground cinnamon, vanilla extract, baking powder and a pinch of salt.
After blending, you can stir in additional ingredients like apple chunks, dried fruits, chocolate chips or chopped walnuts.
Let the batter rest in the fridge for 10 minutes.
Transfer the batter into the prepared baking pan, spreading it out evenly.
Bake in the middle rack of the hot oven for 25-30 minutes, or until the bars have set and are cooked inside. To check for doneness, insert a toothpick into the center: it should come out clean with just a few crumbs.
Let cool for at least 10 minutes before slicing into bars.