For the cake
Preheat the oven to 180°C/gas 4.
Line the base and sides of a 30 x 22cm roasting pan with non stick baking parchment
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 then add the whole eggs and yolk a little at a time beating continuously.
Stir in the chocolate and coffee.
Sift the flour with the baking powder and salt then fold into the mixture with the crushed 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, and then cut into required shapes.