To make the cake layers:

1. Using a mixer with a beater/paddle attachment (not a whisk), beat room temperature butter and sugar on medium speed until pale and creamy, about 2 minutes. Scrape down to the bottom of the bowl once during mixing to make sure no butter or sugar is stuck at the bottom.
2. Add eggs one at a time, mixing on the lowest speed after each addition for 30 seconds. Scrape down to the bottom of the bowl to make sure everything is incorporated before adding the next egg.
3. Add vanilla and mix for about 30 seconds to combine.
4. In a small bowl or measuring jug, mix room temperature buttermilk and oil. Add half to the butter mixture and mix on lowest speed to incorporate.
5. In a bowl, sift flour, baking soda, baking powder and salt. Add half of the flour mixture to the mixing bowl and mix on lowest speed to incorporate, scraping down to the bottom of the mixing bowl to check that batter is evenly mixed. Repeat with the remaining buttermilk mixture and then finish with the remaining flour mixture.
6. Grease a 9x12" sheet pan with non-stick oil spray or butter. Pour the batter into the pan and spread it into the corners. Batter will be thick!
7. Bake at 160ºC or 325ºF for 23 minutes, or until a toothpick inserted into the middle of the cake comes out clean.
8. Leave cake to cool in the pan for 10 minutes. Use a spatula to loosen the cakes from the edges of the pans and then turn onto a wire cooling rack. Leave to cool completely.
9. Cut two circles and two semi-circles out of the cake using a 6 inch cookie cutter (sometimes called a cake circle) or use a 6 inch pan to cut around.

To make the filling:

1. In a pan over medium heat, mix apples with butter, water, and cinnamon and and simmer, stirring occasionally, until soft, about 15 minutes. Leave to cool.

To make the brown sugar buttercream:

1. In a pan over medium low heat, whisk together milk and brown sugar until you can't see any grains of sugar in the mixture. Leave to cool completely.
2. Sift the sugar to remove any lumps and check that the butter is at room temperature - you should be able to slice through it easily with a spatula.
3. In a mixer with a beater (paddle) attachment, mix butter for a few seconds until smooth. Add the cooled brown sugar and milk mixture and mix to combine.
4. Add the powdered sugar and mix for about a minute on low.
5. Scrape down to the bottom of the bowl to loosen any ingredients stuck there. Add the salt, cream cheese, and vanilla and mix to combine.

To put the cake together:

1. Spread some buttercream onto the middle of a plate or cake board and press the first layer of cake onto it to attach it.
2. Wrap a piece of acetate tightly around the cake and secure it with tape and optionally, lower a 6 inch cookie cutter or cake circle down around the outside of the acetate to hold it in place.
3. Spread or pipe a third of the brown sugar frosting onto the first layer of cake and then spoon a third of the apple pie filling on top. Sprinkle a third of the crushed shortbread cookies on top.
4. Place the two semi-circles of cake on top to form a circle of cake. Repeat layering another third of each of the fillings and then place the final cake layer on top, finishing with the final third of the fillings on top.
5. Refrigerate the cake for an hour and then remove the cake circle and peel off the acetate.
6. Let the cake come to room temperature before serving. Store leftovers in the fridge in an airtight container like a Tupperware.