Preheat oven to 350 degrees.
In a large mixing bowl combine: flour, sugar, and salt.
In a saucepan, melt the butter on low heat. Add the cocoa, stirring to combine. Add the hot water and allow the mixture to boil for 30 seconds then turn off heat. Pour the cocoa mixture over the flour mixture, and stir lightly to cool.
Pour the buttermilk (or buttermilk substitute) in a small bowl or measuring cup and add the beaten eggs, baking soda, and vanilla. Stir the buttermilk mixture into the butter/chocolate mixture. Pour into an 18-by-13-inch sheet cake pan/cookie sheet and bake at 350 degrees for 20 minutes.
While cake is baking, make the icing: In a saucepan, melt the butter. Add the cocoa, stir to combine, and turn off the heat. Add the milk, vanilla, and powdered sugar, stirring together. Blend in blender until smooth, about 30 seconds.
Place pecans on cake, most times if I even use pecans, I will do half with pecans and half without. Gently pour icing over cake and be careful not to mix the pecans on the "plain" side if you're doing the same.
Cut into squares with a plastic knife.