Preheat oven to 180°C. Line a standard cupcake pan with paper liners.
In a bowl, sift flour, then set aside.
Using an electric mixer on medium speed, beat the butter until soft and creamy, for about 2 minutes.
Add castor sugar and beat for a further 2 minutes until light and fluffy. Stop to scrape the bowl as needed.
Add in vanilla extract and mix until combined.
Add in the eggs, one at a time. Upon adding final egg, beat until completely combined, for about 1 minute.
Reduce the mixer speed to low and beat in half of the flour mixture until mostly combined.
Add in 1 tbsp of milk, then stir to combine.
Add the remaining flour mixture, mixing until just barely combined. Then add in remaining milk and mix until well combined.
Using a rubber spatula, scrape down the sides and bottom of the bowl.
Fill lined cupcake tin with a heaping 1½ tbsp of batter. Place 2 pieces of apple filling atop the batter. Add another 1½-2 tbsp of batter, followed by another layer of 2 pieces of apple. Lightly press apple down so that the batter spreads out.
This step is optional, but recommended! If dusting cinnamon sugar, mix sugar and cinnamon powder into a separate small bowl and whisk together to combine. Sprinkle cinnamon sugar over the top of each cupcake.
Bake in pre-heated oven for approximately 20-25 minutes until a skewer/cake tester inserted in the center comes out clean. Remove from oven and cool for 5 minutes, then remove the cupcakes from the pan and place on a wire rack to cool completely.
Cupcakes can be stored in airtight container for up to 2-3 days.