Preheat the oven to 350°f /180°c. Grease a 9x13" (20x30cm) nonstick pan and line with parchment paper, extending up towards the sides to help remove the bars once cool.

In the bowl of a stand mixer fitted with the paddle attachment, or using an electric hand mixer in a medium bowl, cream together the butter, sugar, and vanilla on medium speed until light and fluffy.

Add the flour, baking powder, salt, and apple pie spice and mix to combine - the mix will look quite crumbly, but will come together once pressed into the pan.

Remove 620g of the mixture and place into the prepared pan - the remainder will be used for the topping.

Shake the pan around slightly so that the base mixture is evenly distributed over the base of the pan. Using your hands, the bottom of a glass or measuring cup or an offset spatula, press the base mixture evenly into the pan, flattening it down as best you can.

Once your base is flat, add the cooled apple pie filling on top and smooth out with an offset spatula or spoon, making sure that the pieces of apple are relatively evenly distributed.

Top with the remaining base mixture, squeezing each handful in your hand to bring it together slightly then crumbling the dough over the surface of the bars.

Bake the bars for about 45 minutes, until the filling is bubbling and the topping is golden brown. If you find that the topping is browning too quickly, tent the pan with foil.

Remove from the oven and allow to cool completely in the pan. You can either chill them or cut them room temperature. Use the parchment paper to remove the apple pie bars from the pan, and place onto a cutting board. Cut into pieces using a sharp knife. Dust with icing sugar before serving if desired.

Store leftovers in an airtight container at room temperature or in the fridge for up to 5 days. Briefly re warm before serving if desired.