Preheat the oven to 180°C (fan 160°C) and line a 20cm x 20cm square baking tin with baking paper, leaving some overhang on two opposite sides. This overhang acts as handles later.

Make the crust: In a medium bowl, rub the cold butter cubes into the 200g flour, 50g icing sugar, and salt using your fingertips. Work quickly until the mixture looks like coarse breadcrumbs with a few pea-sized butter lumps still visible. You should feel a slight greasiness on your fingers and see pale, sandy-textured crumbs.

Press and bake: Tip the crust mixture into the prepared tin and press it down firmly and evenly with the back of a spoon or your knuckles. The surface should feel compact and smooth, with no loose crumbs. Bake for 18-20 minutes until the edges are just turning a pale golden brown and the centre feels dry to the touch. The kitchen will smell warm and biscuity.

Whisk the filling: While the crust bakes, whisk the eggs and caster sugar together in a separate bowl until they're pale, thick, and frothy—this takes about 1 minute. The mixture should look like a light, airy foam and feel silky between your fingers. Add the lemon juice, zest, 2 tablespoons flour, and vanilla, then whisk again until smooth. You'll notice the mixture thins slightly and smells intensely of fresh lemons.

Bake the filling: As soon as the crust comes out of the oven, pour the lemon filling directly onto the hot crust. The hot base helps the filling set evenly. Bake for 18-20 minutes more. The filling will look set around the edges but will still have a slight wobble in the centre when you gently shake the tin. The top should be a pale, glossy yellow, and the aroma will be sharp and sweet.

Cool completely: Remove the tin from the oven and place it on a wire rack. Let the bars cool to room temperature for at least 1 hour—the filling will firm up as it cools. For the cleanest slices, chill the tin in the fridge for another 30 minutes. The filling should feel firm and bouncy to the touch, not sticky or runny.

Slice and dust: Using the baking paper overhang, lift the entire block out of the tin onto a cutting board. Use a sharp, wet knife to slice into 16 squares, wiping the blade clean between cuts. Dust generously with icing sugar just before serving. The sugar will create a fine, snowy layer that contrasts with the glossy yellow filling.