In a medium mixing bowl, whisk together the flour, baking soda, and salt.
In a 12-inch cast iron skillet, combine the sugar, butter, cocoa powder, coffee and water. Bring to a boil over medium-high heat. Remove from the heat and whisk in the flour mixture. Whisk in the buttermilk, vanilla, and eggs. Bake for 20-25 minutes or until the cake looks set and a toothpick inserted into the center comes out clean.
While the cake is baking, make the frosting. In a small saucepan, combine the milk, butter, cocoa powder, and salt. Bring to a boil over medium-high heat. Remove from the heat and whisk the powdered sugar in, ½ cup at a time. Keep whisking until smooth. Whisk in the vanilla.
As soon as the cake is done, spread the frosting over the top in an even layer, sprinkle with the pecans, and let cool at least 10 minutes or until the frosting sets. Can be served warm, room temperature or cold.