For the chocolate cookies with icing, preheat the oven to 180 degrees. Grease a rectangular baking pan (approx. 25 x 15cm) and line with baking paper.
Melt the chocolate in a water bath. Sift the flour, baking powder and salt into a bowl. In another bowl, cream the butter and sugar until fluffy.
Stir in the eggs, then stir in the melted chocolate and vanilla flavoring. Fold in the chopped nuts and flour alternately.
Pour the batter into the pan and bake for 20 to 30 minutes until the top is firm. Let the cake cool before turning it out of the pan.
For the frosting, melt butter in a saucepan and stir in the powdered sugar and cocoa powder. Mix with 2 tablespoons of water and stir until smooth.
Spread the frosting on the cake and divide the chocolate cookies with frosting into rectangular portions after the frosting has set.