Prepare the Oven and Tin: Preheat your oven to 180°C (160°C fan). Line a 23cm x 33cm (9x13 inch) baking tin with parchment paper, leaving some overhang on the sides to make it easy to lift the bars out later.

Make the Crumb Mixture: In a large mixing bowl, whisk together the 340g plain flour, 200g granulated sugar, baking powder, and salt. Add the cold, cubed butter. Using your fingertips or a pastry cutter, rub the butter into the flour mixture until it resembles coarse breadcrumbs with some pea-sized lumps remaining.

Form the Base: Take out approximately one-third (about 300g) of the crumb mixture and set it aside for the topping. To the remaining two-thirds in the bowl, stir in the beaten egg and vanilla extract until it just comes together. Press this mixture firmly and evenly into the bottom of your prepared baking tin. I find that using the bottom of a measuring cup helps create a nice, flat base.

Bake the Base: Bake the base for 15-18 minutes, or until the edges are just beginning to turn a pale golden colour. Let it cool for a few minutes on a wire rack while you prepare the filling.

Prepare the Lemon Filling: In a separate medium bowl, whisk the 400g granulated sugar and 60g plain flour together to remove any lumps. Add the 4 large eggs and whisk until smooth and well combined. Finally, stir in the fresh lemon juice and lemon zest.

Assemble and Bake Again: Pour the lemon filling evenly over the partially baked base. Sprinkle the reserved crumb mixture evenly over the top of the lemon filling.

Final Bake: Return the tin to the oven and bake for a further 25-30 minutes. The bars are ready when the filling is just set and has a very slight wobble in the centre, and the crumb topping is lightly golden.

Cool Completely: This step is crucial! Let the bars cool completely in the tin at room temperature. Then, transfer the tin to the fridge and chill for at least 2 hours, or preferably overnight. This allows the filling to set fully, ensuring clean slices.

Slice and Serve: Once chilled, use the parchment paper overhang to lift the entire block out of the tin. Place it on a cutting board and slice into 16 bars with a sharp knife. Dust with icing sugar just before serving, if you like.