Preheat the oven to 160 degrees C. Grease and flour a 23cm springform cake tin.
Sift together the flour, cocoa powder, baking powder, bicarb soda and salt.
With an electric mixer, beat the butter until creamy then add the brown sugar and beat until light, about 3 minutes. Reduce speed and add eggs one at a time, beating after each. Add the vanilla.
Add the flour mixture alternately with the Guinness, mixing until just blended. Add pecans, then pour the batter into the cake tin. Bake 70 minutes or until it tests done.