Preheat oven to 350°F. Grease an 11x15-inch jelly roll pan. Place butter, water, and cocoa powder in a large pot and bring to a boil over medium-high heat. Remove from heat and whisk in flour, sugar, and salt. Whisk well until thoroughly combined.

Add sour cream, eggs, and baking soda, and again whisk until thoroughly combined and batter is smooth. Pour batter into prepared pan and smooth top with a spatula. Bake 22 minutes, or until a toothpick inserted in center comes out clean. 

When cake is about 5 minutes from being done, start icing. In a 2-quart saucepan over medium-high heat, bring butter, milk, and cocoa powder to a boil. Remove from heat and whisk in powdered sugar and vanilla, mixing until smooth. Stir in chopped nuts.

When cake is removed from oven, immediately pour hot icing over cake, gently spreading into an even layer. Let icing set about 30 minutes prior to serving. The cake can be served warm or at room temperature.