I recommend using a kitchen scale for this recipe.
Process oats and shredded unsweetened coconut (separately) in a coffee/spice grinder or in a blender.
For the crumble/streusel topping: Simply mix all crumble/streusel ingredients in a small bowl with a fork. Set aside.
Next, peel and chop 1-2 apples (for this recipe you will need 1 1/2 cups of small apple pieces).
Now make "vegan buttermilk" by mixing the plant-based milk with vinegar and aquafaba (chickpea brine). If you don't have a can of chickpeas at home, you can use applesauce instead. Set aside.
Line a muffin tin with liners or grease the inside of each cup with coconut oil (or use a silicone muffin mold as I did) and preheat oven to 375°F (190°C).
In a bowl or food processor mix/process the dry ingredients. Add the wet ingredients and whisk/stir/process until just combined.
Pour the batter into the muffin mold (only 2/3 full). Then add about 1 tbsp of small apple pieces on top. Carefully press the apples into the batter and top with the streusel.
Bake in the oven for about 25 minutes or until a toothpick comes out almost clean (can still be crumbly but not wet!).
Let the muffins cool completely.
Meanwhile, you can make the icing by simply mixing the two ingredients in a small bowl until you have a runny glaze.
Drizzle the icing on top of each muffin and enjoy your delicious apple streusel muffins!