Preheat the oven to 350 degrees Fahrenheit.
In a large bowl, mix together the yogurt, coconut sugar, butter, eggs, and vanilla.
Fold in the oat flour, baking powder, and apple pie spice. Fold in the grated apple.
Spread into a dozen donut pan and bake for 12-15 minutes or until firm and a toothpick inserted into the donut comes out clean.
Let cool to room temperature. Meanwhile, whisk together the maple syrup and powdered sugar until smooth.
Dip each donut partially into the maple frosting so it coats just the top of the donut. Place on a cooling rack, then sprinkle with a pinch of flaky salt.
Repeat with the remaining donuts.
