1. Preheat oven to 160 degrees fan and line/butter your cake tin. You'll only need 1 medium sized tin for this.
2. In a bowl or mixer, add flour, sugar, baking powder and baking soda and mix enough to combine.
3. Chop the apple into smallish chunks and add to the dry mixture.
4. In a separate bowl, add the eggs, vanilla, oil and yoghurt. Whisk lightly just to break up the egg yolks and combine. Don't worry if the oil and yoghurt don't combine nicely at first, just keep whisking.
5. Pour the wet mixture into the dry and whisk/mix till all ingredients are combined. Pour into cake tin and set to one side.
6. Combine the sugar and cinnamon for the crumble topping, and pour in melted butter. Whisk together until a soft, sand like texture forms.
7. Sprinkle the crumble topping over the cake mixture, make sure not to miss the edges.
8. Pop the cake in the oven for 35-45 minutes or until a skewer comes out clean. Transfer to a wire rack and allow to cool.
9. Combine the icing sugar and milk to form a runny icing. Once the cake has cooled enough to touch (looking for warm, not hot) then drizzle the icing over the sponge and allow to cool...if you can resist.