Preheat oven to 350 F Grease and flour a 10x15 rimmed baking sheet or jelly roll pan.
In a small saucepan, melt butter over low heat, add water and 5 tablespoons of cocoa powder. Bring to a boil, then remove from heat. In a medium mixing bowl, combine the flour, sugar, baking soda and salt. Using a mixer, on medium, mix in sour cream and eggs until combined. Set aside. Mix in cocoa mixture until well 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.
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 powdered sugar and vanilla until combined. Pour frosting over warm cake, top with chopped pecans. Allow to cool for icing to set.