Grease and line a 23cm round cake tin with baking paper.
Place the butter and brown sugar into a bowl and cream together with electric beaters on high speed until pale and fluffy.
Add the eggs, golden syrup, vanilla essence and milk and continue to beat on low speed until smooth and creamy.
Sift in the self-raising flour and mix on low speed until just combined (do not over-beat).
Spread half of the cake batter into the prepared tin.
Add half of the apple slices in a thin layer.
Spread over the remaining cake batter and top with the remaining apple slices. Set aside.
In a bowl, use your fingers to rub the plain flour, rolled oats, butter and sugar together until they resemble crumbs (not too fine).
Sprinkle the crumbs over the cake and press down lightly.
Bake in the oven for 60 minutes or until a skewer inserted into the middle comes out clean (it may take a bit longer than 60 minutes depending on your oven).
Allow to cool in the tin for 30 minutes before transferring to a wire rack to cool completely.
Thermomix Method
Preheat oven to 160 degrees celsius (fan-forced).
Grease and line a 23cm round cake tin with baking paper.
Place the butter, brown sugar, eggs, golden syrup and vanilla extract into the Thermomix bowl and mix for 80 seconds, Speed 4.
Add the milk and mix for a further 20 seconds, Speed 4.
Add the self-raising flour and mix for 60 seconds, Interval Speed, Kneading function.
Spread half of the cake batter into the prepared tin. Add half of the apple slices in a thin layer.
Spread over the remaining cake batter and top with the remaining apple slices. Set aside.
Place the plain flour, softened butter and brown sugar into the Thermomix bowl. Mix for 10 seconds, Speed 7.
Add the rolled oats and mix for 5 seconds, Reverse, Speed 7. Sprinkle the crumbs over the cake and press down lightly.
Bake in the oven for 60 minutes or until a skewer inserted into the middle comes out clean (it may take a bit longer than 60 minutes depending on your oven).
Allow to cool in the tin for 30 minutes before transferring to a wire rack to cool completely. Serve warm or cold.
Video
Notes
Recipe TipsTypes of apple - you can use any type of apple you like for this recipe. Personally I prefer pink lady (sweet) or granny smith (slightly more tart). Other sweet varieties include jazz, gala or fuji. Crumble topping - for the crunchiest crumble make sure you use rolled oats (not quick oats), the butter softened at room temperature, and use your fingers to rub crumble mixture together into a combination of small and big chunks (not too fine).Browning - if you find your crumble is browning too much while it’s cooking, simply place a sheet of foil loosely over the top and continue baking.Apple substitutes - for something a little different, check out my apple and raspberry crumble cake version here. Otherwise you can use any type of tinned or sliced fresh fruit you like. Lemon curd is also delicious.Storing the cake - store in an airtight container at room temperature for up to 3 days or wrap well and freeze for up to 1 month. Reheating the cake- if you'd like to reheat your cake after it's cooled, you can do so on a low temperature in the oven or on 50% power in the microwave. It’s delicious served with cream, ice-cream or custard.