Preheat the oven to 165ºC and line the bottom of a 20cm cake tin and grease the sides with non stick baking spray or butter.

In a medium bowl using an electric hand mixer, cream the butter and sugar until soft and fluffy. Add in the vanilla and eggs and whisk again until the eggs are incorporated into the butter mixture. (See notes)

In a small bowl, mix the flour and the baking powder. Fold the dry ingredients into the eggs and butter mixture taking care not to over mix. Spread the cake mixture into the cake tin and top with the thinly sliced apples. Set aside while you make the crumble.

To make the crumble topping, melt the butter in a microwave or over a medium heat in a small sauce pan. Set aside to cool for 5 minutes.

Using a wooden spoon or spatula, mix the flour, sugar and cinnamon in a medium bowl. Pour the melted butter over the flour and mix until a butter crumble is formed.

Scatter this over the top of the apple cake and bake in the preheated oven for about 35 minutes, or until a skewer comes out with only a few crumbs on it.