Preheat the oven to 180C/350F/Gas 4. Line an 8x8-inch square baking pan with foil or parchment paper. Set aside.
Whisk together the flour, baking powder, and salt. Set aside.
Whisk together the butter, sugar, and vanilla. Add to the dry ingredients and mix in with a fork to form a crumbly mixture.
Press half of the mixture into the base of the prepared pan.
For the Apple Filling
Add the apple chunks to a small bowl. Mix in the lemon juice, cornflour, and spices until the apples are fully coated.
Spoon the apples over the base, and spread out evenly. Sprinkle the remaining crumble mixture on top of the apples, pressing it down gently.
Bake for 35 - 40 minutes until golden brown around the edges. Allow to cool completely in the pan on wire rack before cutting into bars. I placed my pan in the freezer for 1 - 2 hours to speed up the cooling.
Video
Notes
Bars stay fresh, stored in an airtight container, at room temperature or in the fridge for up to 1 week. They also freeze well for up to 3 months. Thaw overnight in the fridge before serving.