Break up the chocolate and melt in the Guinness in a saucepan over medium heat.
Stir constantly until dissolved and set aside to cool to room temperature.
Preheat oven to 350°F (180°C)
Grease a Bundt cake pan and dust lightly with flour.
Sift the flour, baking powder, baking soda and salt together in a bowl.
Using a mixer, cream the butter and sugar together until well blended.
Beat in the eggs, one at a time.
Stir the cooled Guinness/chocolate mixture and mix into the other liquid ingredients until smooth.
Gradually add the flour mixture, mixing continually.
Finally stir in the walnut pieces and pour into the Bundt pan.
Bake for 50 - 60 minutes until a skewer inserted into the center comes out clean.
Allow to cool before turning out onto a wire rack.
This cake improves in flavor and moistness after a day or so in covered storage.