Combine the butter, water, and cocoa in saucepan over medium heat; heat until butter melts.
Add sugar, flour, salt, eggs, baking soda, pumpkin pie spice, sour cream, and vanilla; mix well. Pour into a 15x10x1-inch jelly roll pan. (I use the Pampered Chef Bar Pan.)
Bake at 350 for 20 minutes. Make the frosting in the meantime.
For frosting, combine butter and milk in a saucepan; bring to a boil. Add remaining ingredients and mix well with electric mixer. Allow to cool for 15 minutes.
Spread frosting over hot cake; cool and serve!