Whisk together the egg, oil water, vanilla extract and lemon zest together in a bowl.
Whisk together the sugar, flour, baking powder and salt in a separate bowl.
Combine the dry ingredients with the wet ingredients. Add in the blueberries.
Make the crumble, combine the flour and sugar in a small bowl. Add in 2 tbsp of melted butter, add more as needed until the crumbs form. Do not over-mix because you do not want it to become like a dough.
Fill silicone muffin liners about halfway full with the muffin mix. Place a couple of blueberries on top of the muffins (for exploding blueberry effect), then add the crumble mixture on top.
Place in the air fryer basket.
Air fry at 320 °F for 10-11 minutes, or until the interval temperature is around 200-205F or 93-96C. Let the muffins rest in the warm air fryer for a couple of minutes.
Remove from the air fryer basket and silicone liners. Enjoy!