Combine flour and white sugar in a large bowl and set aside.
Put 1/2 cup buttermilk into a large glass and add bicarb soda, stir until combined, set aside.
In medium saucepan over medium heat combine water, 250g butter and 4 tablespoons cocoa. Stir until butter is melted. Allow to boil over medium heat for 5 minutes.
Add chocolate mixture to dry ingredients, mix until well combined. Add buttermilk mixture, stir until blended. Add eggs and mix at medium speed of electric mixer until well blended. Add 1 teaspoon vanilla and mix well. Pour into a 20x30cm pan that has been sprayed with cooking spray. Bake 180 degrees C for 35 minutes.
To Make icing: Combine 125g butter, 1/3 cup buttermilk, and 3 tablespoons cocoa in medium saucepan, allow to come to a boil over medium heat. Remove from heat and add icing sugar and mix well, add 1 teaspoon vanilla and chopped nuts. Pour over hot cake. Allow cake to cool. The longer this sets the better it gets!