Preheat the oven to 325℉. Grease a 13" x 18" rimmed half sheet pan (also called a 'cookie sheet') with softened butter or shortening. Set aside.
In a large bowl (a heat-proof glass bowl), whisk together the all-purpose flour, granulated sugar, baking soda, and salt. Set aside for now.
2 cups all-purpose flour, 2 cups granulated sugar, 1 teaspoon baking soda, ½ teaspoon salt
In a saucepan, over medium-high heat, combine the butter, water, and cocoa powder. Bring the mixture to a boil, once boiling, remove from the heat and immediately pour it into the bowl of dry ingredients and mix together, with a wooden spoon or rubber spatula, until combined.
1 cup (2 sticks) salted butter, 1 cup water, 1/4 cup cocoa powder
Stir in the eggs, buttermilk, and vanilla extract until combined.
2 large eggs, 1/2 cup buttermilk, 2 teaspoons vanilla extract
Pour the chocolate cake batter into the prepared sheet pan. Bake for 20-25 minutes, or until a toothpick inserted into the center of the cake comes out clean.
***While the cake is baking prepare the chocolate frosting - please note that you pour the chocolate frosting over the hot cake, so have it ready to go once the cake is done baking.
In a saucepan, over medium-high heat, add the butter, milk, and cocoa powder. Bring the mixture to a boil. Once boiling, remove the pan from the heat and immediately whisk in the powdered sugar (sifted is preferred) until smooth and well combined.
1/2 cup (1 stick) salted butter, 1/2 cup milk, 1/4 cup cocoa powder, 3 cups powdered sugar
Stir in the chopped pecans and vanilla extract.
1 cup chopped pecans, 1 teaspoon vanilla extract
Immediately pour the warm chocolate frosting evenly over top of the hot cake, and spread it out with a spatula as necessary. Let the cake sit at room temperature until it has cooled (about 30 minutes to 1 hour). Serve slightly warmed or at room temperature.