Preheat oven to 350°F (175°C). Generously grease an 18x13-inch half-sheet pan with non-stick baking spray.

In a large mixing bowl, whisk together the flour, granulated sugar, baking soda, and salt. Set aside.

In a medium saucepan, combine 1 cup of butter, 1/4 cup cocoa powder, and 1 cup of water. Bring to a rolling boil over medium-high heat, whisking until smooth. Remove from heat immediately.

Pour the hot chocolate liquid into the dry ingredients and stir gently until halfway combined.

In a small bowl, whisk the buttermilk, eggs, and 1 teaspoon of vanilla extract. Add this to the batter and stir until perfectly smooth.

Pour the batter into the prepared sheet pan and spread it out evenly. Bake for 20-22 minutes until the center springs back.

During the last 5 minutes of baking, prepare the icing. In a saucepan, bring 1/2 cup butter, 1/4 cup cocoa powder, and the milk to a gentle boil. Remove from heat.

Vigorously whisk the sifted powdered sugar and 1 teaspoon of vanilla extract into the hot liquid until completely smooth and glossy.

Immediately upon removing the cake from the oven, pour the warm fudge icing evenly over the hot cake. Spread gently to the edges.

Sprinkle the chopped pecans evenly over the wet icing. Let the cake cool in the pan at room temperature until the icing sets.