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.