Preheat oven to 180 degrees celsius (fan-forced). Place 12 muffin liners into a 12-hole muffin tray and set aside.
Place the chopped apples into the Thermomix bowl and mix for 5 seconds, speed 5 to grate.
Add the eggs, buttermilk and vegetable oil into the bowl and mix for 20 seconds, speed 4, reverse to combine.
Add the self raising flour, baking powder, ground cinnamon, and caster sugar and mix for 5 seconds, Speed 4 (check the mixture - it should still be a little lumpy).
Scrape down the sides of the bowl and mix for a further 2-3 seconds, Speed 4.
Spoon the mixture into the prepared cases (filling ⅔ of the way to the top).
Decorate with raw sugar, thin slices of apple, oats or seeds - see notes.
Cook for 20-25 minutes or until a skewer inserted into the middle comes out clean.
Allow to cool in the tray for 5 minutes before transferring onto a wire rack to cool completely.