Preheat oven to 350°F. Lightly grease donut pans (with spray, oil, or butter).
In a large bowl, whisk together flour, sugar, brown sugar, baking powder, cinnamon, and salt.
In a medium bowl, mix together eggs, milk, and butter.
Add wet ingredients to the flour mixture, mix until well combined. Fold in grated apple.
Divide the batter between the pans, filling half way. I add the batter to a large zip top bag, cut a hole in the corner and squeeze it into the mold to keep things easy and neat (or use a piping bag).
Bake for 12-14 minutes or until donut springs back when touched and center is cooked.
Cool in the pan for 5 minutes, then gently turn out onto wire rack.
Meanwhile, whisk together powdered sugar, cinnamon, brown sugar, and milk to create the glaze. Add a couple drops of milk if the mixture needs to be a bit thinner.
Dip the top of each donut in the glaze and flip over. Touch up glaze if desired. Allow glaze to harden before serving.
Notes
Store in an airtight container for up to 2 days for best taste and texture.