Preheat oven to 350° F and grease an 18x13 sheet pan with cooking spray or butter.
Combine flour, sugar, baking soda, and salt in a medium bowl. In a small bowl, beat together the sour cream and eggs. Pour the egg mixture into the dry ingredients and mix until a crumbly dough forms.
In a saucepan, melt the butter. Mix in the water and cocoa powder. Bring to a boil, then pour into the crumbly dough mixture. Mix well until a smooth batter forms.
Pour batter into prepared cookie sheet. Bake at 350° F for 20 minutes.
For the frosting:
While cake is in the oven, melt the butter in a saucepan over medium heat. Add the milk and cook over medium heat until combined with the butter.
Add the vanilla, cocoa powder, and sifted powdered sugar. Mix together.
If clumps have formed, then strain the frosting using a fine mesh strainer. Once the frosting is smooth, add the nuts and mix them in.
Pour the warm frosting over the warm cake. Spread evenly.
