Prep. Preheat oven to 350º F. In a large bowl, whisk together the flour, sugar, and salt, set aside. Whisk together the buttermilk, eggs, baking soda, and vanilla extract, set aside.
Make the cake batter: Melt the butter in a large saucepan. Stir in the cocoa powder and espresso powder with the melted butter. Boil for 30 seconds. Remove the saucepan from the heat. Add the flour, salt, and sugar mixture to the cocoa mixture. Stir to combine. Stir in the buttermilk mixture and stir until the cake batter is smooth and well combined.
Bake the cake: Carefully pour the cake batter into a 18x13 rimmed baking sheet. bake in the preheated oven for 20 minutes.
For the Icing:
Make the icing: While the cake is baking, prepare the icing. In a saucepan, melt the butter. Stir the cocoa powder and espresso powder into the melted butter. Remove from the heat. Stir in the milk, vanilla extract, and the powdered sugar. If using pecans, stir those in as well. Pour the mixture over the warm cake when it has baked. Allow to cool for 20 minutes before slicing and serving.