Preheat the oven to 140°C and line a 20 x 20 cm tin with baking paper.
In a mixer with a paddle attachment attached, place the flour, sugar, baking powder, eggs, butter, lemon juice and vanilla essence and mix until combined.
Fold the chopped apple into the mixture.
Pour the mixture into the prepared tin and sprinkle the demerara sugar over the top
Place in the oven and bake for 1 - 1 hr 15 minutes or until a cake tester comes out clean.
Remove from the oven, allow to cool for a couple of minutes before placing on a cooling rack to cool completely.
Cut up into squares and serve