Preheat oven to 375 degrees F. Liberally grease a 15x10" jelly roll pan with cooking spray. Set aside.
Bring butter, water and cocoa powder to a boil in a large saucepan over medium-high heat, stirring until melted and smooth. Remove from heat and add in the sugar and flour, mixing well. Lastly, add in the remaining cake ingredients until blended. Do not over-mix.
Pour the mixture into the prepared pan and bake for approx. 20-25 minutes or until a toothpick inserted near the center comes out clean or with moist crumbs. Set aside.
Meanwhile, while cake partially cools, prepare your frosting. In another large saucepan, melt together the butter, cocoa powder and milk. Bring to a boil, stirring constantly, then remove from heat and add in the powdered sugar, vanilla and chopped pecans. Working quickly, pour the frosting over top of the warm cake, spreading into an even layer. Allow the frosting and cake to cool before cutting into slices.