Preheat your oven to 350°F (175°C). Grease and flour an 18x13-inch half-sheet pan or jelly roll pan.
In a large mixing bowl, combine flour, granulated sugar, and salt. Set aside.
In a medium saucepan, combine 1 cup butter, water, and 4 tablespoons cocoa powder. Bring to a boil over medium heat, stirring occasionally.
Once boiling, remove from heat and pour the hot chocolate mixture over the dry ingredients. Stir until just combined.
In a separate bowl, whisk together eggs, buttermilk, baking soda, and 1 teaspoon vanilla extract.
Add the egg mixture to the chocolate batter and stir until smooth.
Pour batter into the prepared pan and spread evenly to the edges.
Bake for 18-22 minutes, or until the cake springs back when lightly touched and a toothpick inserted into the center comes out clean.
About 5 minutes before the cake is done, begin making the frosting. In a medium saucepan, combine 1/2 cup butter, 4 tablespoons cocoa powder, and milk. Bring to a boil over medium heat, stirring constantly.
Remove frosting from heat and whisk in powdered sugar and 1 teaspoon vanilla extract until smooth. Stir in pecans if using.
Immediately pour warm frosting over the warm cake as soon as it comes out of the oven, spreading evenly across the entire surface.
Allow the cake to cool completely in the pan, about 60 minutes, before cutting and serving.
