Preheat your oven to 350°F. Grease a 10x15-inch jelly roll pan with butter or cooking spray and set it aside.
Whisk together the 2 cups of flour, 2 cups of sugar, and 1/2 teaspoon of salt in a large mixing bowl until they are fully combined and free of lumps.
Combine 1 cup of water, 1/2 cup of butter, 1/2 cup of oil, and 1/4 cup of cocoa powder in a medium saucepan. Bring the mixture to a boil over medium heat, stirring frequently, then immediately remove it from the heat.
Pour the hot cocoa mixture over the dry flour and sugar mixture. Use an electric mixer on medium speed to beat the ingredients together until they are just combined, which should take about 1 minute.
Add the 1/2 cup of buttermilk, 2 eggs, 1 teaspoon of baking soda, and 1 teaspoon of vanilla extract to the batter. Beat on medium speed for 1 more minute until the batter is smooth and uniform.
Pour the finished batter into your prepared pan, spreading it into an even layer. Bake for 20 minutes, or until a toothpick inserted into the center comes out clean.
Begin making the frosting 5 minutes before the cake is done baking. Melt 1/2 cup of butter with 1/4 cup of cocoa powder and 1/3 cup of buttermilk in a saucepan over medium heat, stirring until smooth, then bring it just to a boil. Immediately remove from heat.
Gradually whisk the 4 cups of powdered sugar and 1 teaspoon of vanilla extract into the hot frosting until it is completely smooth and free of lumps. Stir in the 1 cup of chopped pecans. Pour the warm frosting over the hot cake as soon as it comes out of the oven, spreading it evenly to the edges.