Preheat the oven to 200C/180C Fan/Gas 6. Line an 8" square baking tin.
Prepare the apples: peel and chop the apples into a 2 cm dice (chunky enough to keep some texture but not too big).
Heat the apples, butter, brown sugar, cinnamon and about 2 tbsp water in a saucepan over a medium heat. Stir until the apples have softened - but before they turn 'mushy'.
Let the apples cool slightly while you make the biscuit layers. Do this by first combining the flour, sugar, baking powder in a large bowl, or the bowl of a stand mixer, on a medium speed.
Add in the egg, butter and vanilla and combine again to form a cookie dough. Press 2/3rds of this into the pan, forming an even layer.
Pour the apples (drain off any excess moisture if there is any) over the shortbread. Then sprinkle the remaining shortbread over the apples. Bake for 35-40 minutes, until golden brown on top. Let cool on a wire rack.