1. Preheat the oven to 160C/325F/Gas mark 3. In a bowl, beat the butter and sugar together until soft and creamy. Add the flour and salt and sieve over the cocoa powder. Carefully mix together and then beat until well combined.
2. Press into an 18 x 28-cm (7 x 11-in) oblong cake tin. Prick all over with a fork and bake in the oven for about 1 hour until firm. Cut into 24 fingers while warm and allow to cool in the tin.
Dredge with a little caster sugar and a light dusting of cocoa powder to serve, if liked.