Start with your cookie dough. Mix the flour, bicarbonate of soda, cornflour and salt in a bowl. Then in another bowl mix the melted butter with the sugar until the sugar has dissolved. Then add the egg and vanilla extract and mix. Combine the flour mixture with the butter/sugar mixture. This should form a dough. Roll it into a sausage approx. 4cm in diameter. Wrap in cling film and refrigerate for at least 2 hours.
Whilst the dough is refrigerating, peel your apples and chop them into small cubes. Mix with the lemon juice (this prevents the apples from going brown) and stir in the light brown soft sugar. Leave to one side. This will draw some of the moisture out of the apples.
In another bowl place the butter and flour. Rub the flour into the butter with your fingertips until you have rough breadcrumbs. Mix in the porridge oats and demera sugar and place in a thin layer on a baking sheet. Leave to one side.
Once the cookie dough has chilled, pre-heat your oven to 160c.
Take the cookie dough out of the fridge and slice into 20 rounds. Flatten them between your hands a little and then place onto a couple of non-stick baking sheets. Make sure there’s room between the cookies as they will spread during cooking.
Divide the chopped apples and spoon in a heap on top of the cookies. Discard the juice that has been drawn out from the apples.
Place the cookies and the crumble mixture in the oven for 11 minutes.
Take the cookies out, increase the temperature to 190c and let the crumble mixture cook for a further 7-8 minutes (keep an eye on it to ensure it doesn’t burn)
Take the crumble mixture out of the oven, then remove the cookies from the trays using a spatula whilst they’re still a little warm (they may stick if you leave them on the trays for too long