Preheat the oven to 160ºc and line a 23cm square baking tin with nonstick baking paper.
Peel, core and chop the apples. Toss them in a bowl with some sugar and cinnamon and set aside while you make the batter.
Use an electric hand whisk to cream the butter and sugar until they are very light and fluffy. You’re looking for the consistency of thick mayonnaise with the butter and sugar mix.
Add in the eggs one at a time mixing after each addition. Your cake mix may start to curdle a little bit. Don't worry about that even a little bit. The cake batter comes back together as soon as you add in the flour.
Add in the flour, baking powder and cinnamon. If you're using self-raising flour, then skip the baking powder. Fold it all in gently with a wooden spoon. Spread the cake mix into the lined baking tin. Spread the apples evenly over the top of the cake and bake in the preheated oven for about 35-40 minutes. *see notes
If you're using the demerara sugar then sprinkle a tablespoon of it over the hot apple cake.
Notes
If you’re using self raising flour in place of plain flour, then skip the baking powder and use 350g self raising.
The flour, baking powder (if using) and cinnamon don’t need to be sifted together but you can if your flour is lumpy.
Sprinkle the demerara sugar over the baked cake rather than before hand so that the sugar doesn’t melt of burn and you maintain the crunch from the sugar.
How to tell when a cake is done- the cake will be spongey but bounce back when very gently pressed in the centre. Inserting a skewer or toothpick should come out with a few moist crumbs sticking to it. If the skewer is clean, then you will very likely end up with a dried and over baked cake.