Preheat the oven to 350 degrees and lightly grease an 18x13 inch pan with nonstick cooking spray.
Heat the butter in a small saucepan over medium heat.
When the butter is melted, add the water and unsweetened cocoa powder. Wisk to combine. Bring the mixture to a boil, then remove the pan from heat and allow to cool slightly.
While the chocolate mixture is cooling, whisk together the flour, sugar, baking soda, and salt. Add the vanilla, buttermilk, and eggs. Beat to combine.
Add the chocolate mixture and beat until smooth.
Spread the batter into the prepared pan and bake for 20 to 25 minutes, until the cake bounces back when pressed with your finger.
While the cake is baking, heat the butter in a saucepan over medium heat. When the butter is melted, whisk in the milk, cocoa powder, and vanilla extract.
Stir in the powdered sugar until smooth, then fold in the chopped pecans.
Pour the icing over the top of the warm cake. Allow the cake to cool before slicing.