Preheat oven to 180°C (160ºC fan). Line a 12-hole standard muffin tin with paper liners.
Combine flour, baking powder, cinnamon, sugar and a pinch of salt in a large bowl.
In a large bowl whisk together oil, egg, milk and vanilla. Pour the liquid ingredients into the dry ingredients and mix until just combined. Fold the diced apple through batter then divide between liners.
To make the crumble, combine flour and butter in a bowl. Use fingers to rub the butter into the flour until the mixture resembles breadcrumbs. Stir through the sugar. Sprinkle crumble evenly over the muffins then bake for 30–35 minutes or until cooked when tested with a skewer.
Remove from tin and cool on a wire rack before drizzling each muffin with 15g (1 tbsp) Nutella®, to serve.