Preheat your oven to 160 degrees Celsius (if it has a fan, use that setting).

Prepare a 23cm round cake tin by greasing it and lining it with baking paper.

In a bowl, combine the butter and brown sugar. Use an electric beater on high speed until it's light and fluffy.

Add the eggs, golden syrup, vanilla essence, and milk to the mixture. Keep using the beater on low speed until everything becomes smooth and creamy.

Sift in the self-raising flour and mix on low speed just until it's combined. Don't overmix.

Spread half of this cake batter into your prepared tin.

Place half of the apple slices in a thin layer on top of the batter.

Now, spread the remaining cake batter over the apples.

Top it off with the remaining apple slices. Set it aside.

In another bowl, use your fingers to mix together the plain flour, rolled oats, butter, and sugar until you get crumbs. Don't make them too fine.

Sprinkle these crumbs over the cake and press them down lightly.

Put the cake in the oven and bake for around 60 minutes. Check it by inserting a skewer into the middle; it should come out clean. It might need a bit more time, depending on your oven.

After baking, let it cool in the tin for 30 minutes before transferring it to a wire rack to cool completely.

Enjoy your delicious Apple Crumble Cake!