Preheat the oven to Gas Mark 4.
Sieve together the flour and salt.
Melt the butter and chocolate together in a basin over hot water.
Place sugar in a bowl, add eggs and essence and mix well.
Beat in the chocolate mixture.
Stir in the flour and nuts and place in a 27.5 x 18cm (11 x 7 inch) greased tin.
Bake on the third shelf for thirty minutes.
Cool in the tin and cut into squares.