Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 10x15 inch pan.
Combine the flour, sugar, baking soda and salt. Beat in the sour cream and eggs. Set aside. Melt the butter on low in a saucepan, add the water and 5 tablespoons cocoa. Bring mixture to a boil then remove from heat. Allow to cool slightly, then stir cocoa mixture into the egg mixture, mixing until blended.
Pour batter into prepared pan. Bake in the preheated oven for 20 minutes, or until a toothpick inserted into the center comes out clean.
For the icing: In a large saucepan, combine the milk, 5 tablespoons cocoa and 1/2 cup butter. Bring to a boil, then remove from heat. Stir in the confectioners' sugar and vanilla, then fold in the nuts, mixing until blended. Spread frosting over warm cake.