Melt the butter and add to the caster sugar in a large mixing bowl. In a separate bowl lightly whisk the eggs. Add to the butter and sugar and mix together. Sift in the cocoa, flour and salt and mix. Chop the two chocolate blocks into small pieces and fold through the mix. Pour into prepared tin. Bake for approximately 25 – 35 minutes. Allow to cool in the tin.
To serve, cut into 32 small squares and dust with icing sugar.