For the muffins:
For the crumble:
1. Preheat oven to 350F (175C). Line a 12-cup muffin pan with muffins liners, spray the cups with oil (optional) and set aside.
2. Make the muffins: In a bowl mix milk, oil, egg and vanilla extract. Set aside.
3. In a separate bowl, combine flour, baking powder, sugar and salt.
4. Peel the apples and cut into small chunks. Place apple chunks into the flour mixture and toss well to ensure the apples are well coated.
5. Pour the wet mixture into the dry mixture and mix until combined.
6. Scoop the batter into muffin cups until 3/4 full.
7. Make the crumble: Combine the ingredients for the crumble topping together in a small bowl and crumble together using your finger tips.
8. Sprinkle some crumble on top of each muffin batter. Bake for 20-25 minutes, until a toothpick inserted into the center comes out clean.
9. Let cool before serving.