Preheat oven to 350.
In a large bowl, sift together the flour, cocoa powder, salt, and baking soda. Set aside.
In a separate bowl, combine the butter, sugar, shortening, eggs, and vanilla extract with a hand or stand mixer.
Add the hot water and sour milk to the butter mixture, and combine well.
In stages, add the bowl of dry ingredients to the wet bowl, mixing well after each addition.
Line a baking sheet with parchment paper. Drop about 3 tablespoons of batter onto the sheet, leaving 2" of space between each shell.
Bake 10 minutes, and allow to cool several minutes on the sheet before removing to a rack to finish cooling.