Preheat and Prepare: Preheat your oven to 180°C (160°C fan). Line a 20x20cm baking tin with parchment paper, leaving an overhang on two sides. This creates 'handles' for lifting the bars out later, keeping your fingers clean.

Make the Shortbread Crust: In a large bowl, beat the softened butter and 100g caster sugar together until pale and fluffy — the mixture should look like a light, sandy paste. Stir in 250g flour and the salt until a soft dough forms. Press this dough evenly into the base of your prepared tin. You'll feel it should be firm but not sticky.

Blind Bake the Base: Prick the dough all over with a fork to stop it bubbling. Bake for 18-20 minutes, or until the edges are a pale golden brown and the centre looks dry and set. The smell should be nutty and buttery.

Whisk the Lemon Filling: While the crust bakes, whisk the eggs in a large bowl until frothy — you'll see small bubbles forming on the surface. Add the remaining 300g sugar, lemon juice, and lemon zest. Whisk again until combined. In a separate small bowl, mix the 60g flour and baking powder, then sift this into the wet mixture. Whisk until smooth; the batter will look thin and pale yellow.

Pour and Bake: Once the crust is ready, pour the lemon filling over the hot base. The filling will sizzle slightly as it hits the warm tin. Return the tin to the oven and bake for 18-22 minutes. The filling is done when it is set around the edges but still has a slight jiggle in the centre — like a firm jelly. The top will be a pale, matte yellow.

Cool Completely: Remove the tin from the oven and place it on a wire cooling rack. Let it cool to room temperature for about 30 minutes, then transfer it to the fridge for at least 2 hours. This cooling process is crucial; the bars will be too soft to slice cleanly if you rush it.

Slice and Serve: Lift the block out using the parchment overhang. Dust generously with icing sugar. Using a sharp, wet knife, cut into 16 squares. The bars should be firm, with a clean cut that reveals a bright, glossy yellow filling against a crumbly, golden crust.