Preheat the oven to 350°F (175°C). Grease and flour an 18x13-inch half sheet pan (jelly roll pan) and set aside.

In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking soda, and salt until evenly combined. Set aside.

In a medium saucepan over medium heat, combine the butter, water, and cocoa powder. Stir continuously until the butter melts completely and the mixture comes to a gentle boil. Ensure the mixture is smooth and free of lumps.

Immediately pour the hot chocolate mixture over the dry ingredients. Stir until partially combined.

Add the buttermilk, eggs, and vanilla extract to the mixture. Stir gently until the batter is smooth and just combined. Do not overmix.

Pour the batter into the prepared sheet pan and spread evenly using a spatula. Bake for approximately 20 minutes, or until a toothpick inserted into the center comes out clean.

While the cake is baking, prepare the poured chocolate icing. In a saucepan over medium heat, combine the milk, butter, and cocoa powder. Stir until the butter melts and the mixture begins to simmer.

Remove from heat and stir in the vanilla extract. Gradually whisk in the sifted powdered sugar until the frosting is smooth and glossy. If desired, stir in finely chopped pecans.

Remove the cake from the oven and allow it to cool for approximately 5 minutes. Pour the warm icing evenly over the warm cake and spread gently with a spatula to ensure full coverage.

Allow the frosting to set at room temperature for 20–30 minutes before slicing. For a firmer finish, allow the cake to cool completely or refrigerate briefly.