In a large mixing bowl, combine the flour and the sugar. Set aside.
In a medium saucepan, combine butter, shortening, water, and cocoa. Stir and heat to boiling.
Pour boiling mixture over the flour and sugar in the bowl.
Add the buttermilk, eggs, baking soda, and vanilla to the mixing bowl. Mix well using a wooden spoon or high speed on an electric mixer.
Pour into a well-greased baking sheet (17.5" x 11") pan.
Bake at 400 degrees for 15-18 minutes or until brownies test done in center.
To make the frosting, mix cream cheese and butter together with a mixer until completely incorporated. Add in powdered sugar and peanut butter and mix well. If it's too thick, thin it out with a little bit of milk. If it's too thin, add some more powdered sugar.
Spread frosting on cooled brownies.
If desired, drizzle melted chocolate chips over frosting and sprinkle Reese's pieces candies on top.