Step 1: Preheat your oven to 350°F (175°C) and grease a 18x13 inch jelly roll pan.
Step 2: In a large mixing bowl, whisk together the flour, sugar, baking soda, and salt.
Step 3: In a medium saucepan, combine 1 cup of butter, water, and 4 tablespoons of cocoa powder. Bring to a boil, then remove from heat.
Step 4: Pour the hot chocolate mixture over the dry flour mixture and stir until well combined.
Step 5: In a small bowl, whisk together the buttermilk, eggs, and 1 teaspoon of vanilla extract. Stir this into the chocolate batter until smooth.
Step 6: Pour the batter into the prepared jelly roll pan and bake for 18-22 minutes, or until a toothpick comes out clean.
Step 7: Five minutes before the cake is done, prepare the icing. In a saucepan, combine 1/2 cup butter, 4 tablespoons cocoa, and 6 tablespoons milk. Bring to a boil.
Step 8: Remove icing from heat and whisk in the powdered sugar and 1 teaspoon of vanilla until smooth. Stir in the chopped pecans.
Step 9: Pour the warm icing over the hot cake as soon as it comes out of the oven. Spread evenly and allow to cool before slicing.