Preheat and prepare the tray: Set your oven to 180°C (160°C fan). Line a 20x20cm square baking tray with parchment paper, leaving an overhang on two opposite sides — this creates handles to lift the bars out later. The paper should feel crisp and secure against the tray walls.

Make the shortbread crust: In a large mixing bowl, beat the softened butter, 200g plain flour, 50g icing sugar, and salt together until the mixture resembles fine breadcrumbs and holds together when pressed between your fingers. You'll notice the dough turning from pale yellow to a uniform, buttery colour — that's your cue it's ready. Press the mixture firmly and evenly into the prepared tray, making sure the surface is smooth and level. The crust should feel compact and slightly oily to the touch.

Blind bake the crust: Prick the crust all over with a fork — you'll see small steam holes forming. Bake for 18-20 minutes, until the edges are just starting to turn a light golden brown and the centre feels dry to the touch. The kitchen will smell richly of toasted butter and shortbread. Remove from the oven and set aside to cool for 5 minutes while you prepare the filling.

Whisk the lemon filling: In the same bowl (no need to wash it), whisk the eggs until they're frothy and pale yellow — you'll see small bubbles forming on the surface. Add the caster sugar and whisk for another minute until the mixture looks thick and glossy. Sift in the 60g plain flour and whisk until smooth, then add the lemon juice, lemon zest, and vanilla extract. The mixture should be thin and bright yellow, with the zest flecked throughout like tiny specks of sunshine.

Pour and bake: Pour the lemon filling over the warm shortbread crust. Gently tilt the tray so the filling spreads evenly to all corners — you'll see it settle into a smooth, liquid layer. Bake for 20-25 minutes, until the filling is set but still has a slight wobble in the centre when you gently shake the tray. The edges should be pale golden and the surface will look matte rather than glossy. You'll notice the scent shifting from buttery to bright and citrusy.

Cool and slice: Place the tray on a wire cooling rack and let it cool completely at room temperature — about 1 hour. The bars will firm up as they cool, and the surface will become smooth and slightly firm to the touch. Once cool, use the parchment paper handles to lift the whole block out of the tray. Dust generously with icing sugar, then slice into 16 squares with a sharp knife. For clean edges, wipe the knife between each cut.