Prepare the Oven and Pan: Preheat your oven to 180°C (160°C fan). Line a 20x20cm square baking tin with parchment paper, leaving an overhang on two opposite sides. This will act as your handles to lift the bars out later. The paper should feel snug against the sides of the tin.

Make the Shortbread Crust: In a medium bowl, cream the softened butter and caster sugar together with a wooden spoon until pale and fluffy. The mixture should look light and feel smooth, not grainy. Add the flour and salt, and mix until a soft, cohesive dough forms. It will be slightly crumbly but should hold together when pressed.

Press and Bake the Crust: Transfer the dough into your prepared tin. Press it evenly into the bottom using your fingertips or the flat bottom of a glass. The surface should be flat and compact, with no visible cracks. Bake for 18-20 minutes, or until the edges are just turning a pale, golden brown. The centre will still feel slightly soft but will firm up as it cools. You should smell a rich, buttery aroma.

Whisk the Lemon Filling: While the crust bakes, make the filling. In a large bowl, whisk the eggs and granulated sugar together until well combined and slightly thickened, about 1 minute. The mixture should look pale and feel smooth. Whisk in the flour, salt, lemon juice, and lemon zest until completely smooth. The batter will be thin and smell intensely of fresh lemons.

Pour and Bake the Bars: Pour the lemon filling over the warm, pre-baked crust. The warm crust helps the filling set evenly. Bake for 20-25 minutes. The filling should be set around the edges but still have a slight jiggle in the very centre when you gently shake the pan. The top will look matte, not wet or shiny, and the surface will feel firm to a light touch. A faint, glossy sheen is a sign it is done.

Cool and Chill Completely: Allow the bars to cool to room temperature in the tin on a wire rack. Once cool, transfer the tin to the refrigerator and chill for at least 2 hours, or up to overnight. This step is crucial for clean, neat slices. The bars should feel completely cold and solid to the touch.

Slice and Serve: Using the parchment overhang, lift the entire block of lemon bars out of the tin and onto a cutting board. Using a sharp, non-serrated knife, slice into 16 squares. For the cleanest cuts, wipe the knife blade clean between each slice. Dust generously with icing sugar just before serving.