Pre-heat oven to 180°C/ 357F. Prep your cupcake cases. Place 7 into a muffin tray.
In a large bowl, sieve + combine all dry ingredients (flour, baking powder, baking soda, salt and cinnamon) together. In a separate bowl, combine the milk and the apple cider vinegar. Once curdled, add the other wet ingredients: the oil and vanilla extract. Add the dry ingredients to the wet ingredients, along with the flax egg and grated apple. Whisk until all combined.
Place the batter into the 7 cupcake holders. Bake for 20 minutes and then leave to cool.
Once you have removed your cupcakes, keep your oven on to make your crumble. In a small bowl, place the flour (18g), butter (9g) and sugar (10g). Combine with your fingers until a fine crumble has formed. Place onto a lined baking tray and bake for 18 minutes, until golden and crispy. Then, remove from the oven to cool.
In a standing mixer, add the butter and beat on medium-high speed until creamed (2-3 minutes). Sieve the icing sugar into the same bowl. Then add the vanilla extract. Mix on a low speed for 1 minute, and then increase to a high speed for a further minute or two, until a light and smooth buttercream icing has formed. You can leave this for longer if you want the icing fluffier. If you have made this ahead of time and the cupcakes aren't cool yet, make sure to place this in the fridge until you use it.
Once your cupcakes are completely cool, place the icing into a piping bag and pipe onto the cupcakes. You can do this in whatever way you like! Then, evenly place some of the cinnamon coated chopped apple on top of the buttercream icing. Then sprinkle some crumble on top of the apple. And that's it! I hope you love them!