Heat the oven to 150°C (130°C fan). Line a 20 × 30 cm slice tin with baking paper.
Beat the butter and sugar together until just combined and smooth. Don’t beat air into it — you’re not making a cake.
Sift in the flour, cornflour and salt, and mix on low or by hand only until it comes together into a soft dough. Stop as soon as there’s no loose flour.
Press the dough evenly into the tin, smoothing the top with the back of a spoon. Prick all over with a fork, in rows, right through to the base.
Bake for 30–35 minutes. It’s ready when the edges are pale gold and the centre is set but barely coloured. Do not let it brown all over.
Dust with caster sugar while hot, then cut into 24 fingers **while still warm** — it will shatter if you wait until it’s cold.
Leave in the tin to cool completely before lifting out. It firms up considerably as it cools.