Preheat the oven to 180°C/gas 4.
Line the base and sides of a 30 x 22cm roasting pan with foil.
Melt the chocolate in a double boiler or in a bowl set over, but not touching , hot water.
While the chocolate is melting, cream the butter and sugar until light and fluffy. Add the whole eggs and yolk with the beaters still going.
Stir in the chocolate and coffee.
Sift the flour with the baking powder and salt. Fold into the mixture together with the walnuts.
Spoon the mixture into the prepared tin. Bake for 20-25 minutes or until a skewer inserted into the mixture comes out a bit sticky. The interior should resemble fudgy goo, not a sponge.
Leave to cool in the pan, then cut into squares.