Put the chocolate pieces in a heatproof bowl and stand over a pan of hot water until melted, stirring occasionally. Spoon the chocolate into a greased 19 cm (7 1/2 in) square cake tin. Spread it out over the bottom and leave to set.

Meanwhile, cream together the butter and sugar until the mixture is light and fluffy. Beat in the egg thoroughly. Mix together the remaining ingredients and add to the creamed mixture. Spoon into the tin and spread over the set chocolate.

Bake in the centre of a cool oven (150°C/300°F or Gas Mark 2) for 40 to 45 minutes, or until golden-brown. Remove from the oven and leave for 5 minutes, then carefully mark into 12 to 16 squares with a sharp knife. The mixture will be quite sticky at this stage.

Leave until cold, then loosen with a palette knife and lift each square carefully from the tin.