Pre-heat the oven to 180C.
Place the butter and sugars in a freestanding mixer and beat until paler in colour and fluffy.
With the motor running, add the eggs one at a time, followed by the milk.
In a separate bowl, measure out the flour, baking powder, and salt, and stir a little to combine before adding to the other ingredients in the mixer. Give a final whizz, then set the batter aside.
Peel, core, and slice the apples as finely as possible.
Take a lined 10” square tin and pour the batter into the bottom, spreading evenly.
Top with the apples and sprinkle with the Demerara and cinnamon.
Bake for 25 minutes.
Whilst the cake is cooking, make the crumble topping by rubbing the butter and flour together until the mixture resembles breadcrumbs.
Add in the oats, sugars, and cinnamon and stir together well.
After the 25 minutes of baking are finished, remove the cake from the over na d spread one the crumble topping. Try to be as brief as possible.
Return the cake to the oven for a further 25 minutes, covering with parchment for the final 10 minutes to stop it browning too much.
Remove from the oven and leave to cool for 10 minutes in the tin before turning out with care and leaving to cool completely.
Slice into 16 pieces.
