Heat the oven to 180C/160C fan/gas 4. Line a 18 x 28cm baking tin with baking parchment. Put the butter, flour, 150g of the brown sugar and the vanilla extract into a food processor with 1 tsp salt, and blitz until the mixture comes together. Press two-thirds of the dough into the tin in an even layer, then bake for 12-15 mins until golden. Keep the remaining dough chilled while you prepare the apples.

Tip the apples into a pan with 3 tbsp of the brown sugar, 1 tsp of the cinnamon (if using) and the lemon juice. Cook over a medium heat for 15-20 mins until the apples have softened but not broken down.

Tip the apples over the pastry base, then break up the remaining dough and dot it over the top. Bake for 15-20 mins until the topping is cooked and golden. Remove from the oven and leave to cool in the tin.

Combine the icing sugar and remaining 1/4 tsp cinnamon with enough water to create a thick icing (you need around 2 tsp). Drizzle the icing over the apple pie bars, then slice into 12-16 squares. Will keep in an airtight container for up to three days.