Dice one apple at a time into small chunks. Once you dice the first apple, set it aside. Each apple is used for different steps!
Add all ingredients, including one diced apple, to a high speed blender or food processor. Blend until fully combined (you may have to scrape the sides a couple of times).
Stir in the diced chunks of the second apple.
Spoon batter into each lined muffin tin. Smooth out the top of each muffin with a spoon or small spatula.
If you're making the crumble topping - add ingredients in a small bowl and mix with hands until you reach a sand-like consistency. Place on top of each muffin.
If you're not making the crumble topping - sprinkle extra cinnamon and sugar on top of each muffin.
Bake 28-35 minutes, checking for doneness at 30 minutes.
Once out of the oven, immediately take out of the pan and place on a cooling rack.
Once cooled, if you're making the icing - mix powdered sugar and milk until you reach desired consistency. Top each muffin.