Preheat and prepare your pan. Preheat the oven to 180°C (160°C fan). Line a 20cm x 20cm square baking tin with a long strip of parchment paper, leaving an overhang on two opposite sides. This will be your sling to lift the bars out later. Lightly grease the paper.

Make the shortbread crust. In a large bowl, combine the cold cubed butter, 100g caster sugar, 250g plain flour, and the salt. Use your fingertips to rub the butter into the dry ingredients until the mixture resembles coarse breadcrumbs. You'll feel the butter pieces become smaller and the mixture turn sandy and cool. Alternatively, pulse in a food processor until it just starts to clump together.

Press and blind-bake the base. Tip the crumb mixture into the prepared tin. Press it down firmly and evenly with your fingers or the flat bottom of a glass. The surface should look smooth and compact, with no gaps. Bake for 18-20 minutes, until the edges are just beginning to turn a pale, nutty gold and the surface feels dry and firm to the touch. The scent of toasted butter will fill your kitchen.

Whisk the lemon filling. While the crust bakes, whisk the eggs in a large bowl until they are frothy and slightly paler, about 1 minute. Add the 300g caster sugar and whisk until thick and combined, about 2 minutes more. The mixture should look like a pale, ribbony cream. Whisk in the lemon juice and zest—the aroma will be intensely floral and sharp.

Incorporate the dry ingredients. Sift the 60g flour and baking powder together over the wet mixture. Whisk gently but thoroughly until no streaks of flour remain. The batter will be thin and smooth, like a very loose custard.

Pour and bake. Pour the lemon filling over the hot, pre-baked crust. The crust will sizzle slightly—that's a good sign. Bake for 20-25 minutes. The top should be set and no longer jiggly in the centre, but it will still have a slight wobble when you shake the tin. The edges will be a very light golden brown, and the surface will look glossy and taut, like polished glass.

Cool completely. Place the tin on a wire cooling rack and let it cool to room temperature, about 1 hour. Then, transfer it to the refrigerator and chill for at least 2 hours, or overnight. This step is non-negotiable for clean, sharp slices. The bars should feel firm and cold to the touch.

Slice and serve. Use the parchment overhang to lift the entire block out of the tin. Place it on a cutting board. Using a long, sharp knife, slice into 16 squares. Wipe the knife clean between each cut for pristine edges. Dust generously with icing sugar just before serving.