Oven at 180C/fan 160C/gas 4. Butter a rectangular baking pan (approx 27cm by 20cm) and line it with parchment paper.
The lemon juice should be squeezed over the apples after they have been cored and thinly sliced. Place the apples on a plate. Now Mix the butter, caster sugar, and eggs in a large bowl. Add the flour, baking powder, vanilla, and eggs and beat well.
Now Place half of the mixture in the prepared tin. Place half of the mixture on top, and then repeat the process. Sprinkle the demerara sugar on top.
Bake for 45-50 minutes until the cake is golden and springy to touch. Let cool in the pan for 10 minutes, then take out of the tin and peel off the paper. Slice into squares or bars.