Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 10x15 inch pan. Alternatively, line a 10x15 pan with parchment paper.
Melt the butter on low in a saucepan, add the water and 5 tablespoons cocoa. Bring mixture to a boil then remove from heat. Set aside.
In a small bowl, whisk together sour cream and eggs.
Combine the flour, sugar, baking soda and salt in the bowl of a stand mixer. Beat in the sour cream and egg mixture. Once the cocoa powder mixture is cool enough to touch, beat it into the four mixture on low speed. Scrape sides and beat just until fully mixed.
Pour batter into prepared pan. Bake in the preheated oven for 22-25 minutes, or until a toothpick inserted into the center comes out clean.
About half way through the cooking process, make the frosting.
To make the frosting, combine milk, additional 5 tablespoons cocoa powder, and additional 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.