Lightly grease and line a 30cm x 20cm baking tin with baking paper.
With an electric mixer beat the butter and brown sugar until light and fluffy. Now add the eggs one at a time and mix in well for another minute. On low speed, mix through the cocoa powder and flour. Add the chopped Fantales and walnuts and stir through evenly. Pour the mixture into the baking tin and sprinkle with the salt.
Cook in the oven for 30 minutes or until an inserted skewer comes out clean. Allow to cool slightly before turning out and cutting into squares. Dust with cocoa if you wish.