Grease one 13 x 9 inch pan. Set aside.
Preheat oven to 350.
In a medium bowl combine the flour, cocoa powder, salt, baking soda, baking powder, mix and set aside.
In bowl of a mixer, beat butter on high speed for 30 seconds.
Add sugar and brown sugar, beat until combined.
Scrape sides of bowl and beat another 2 minutes.
Add eggs one at a time, beating well after each addition, then add vanilla.
Add flour mixture and buttermilk alternately to sugar egg mixture, on low speed just until combined.
Beat an additional 20 seconds.
Pour into prepared pan and bake for 40-45 minutes.
Check after 35 minutes as your oven may run hotter than mine.
Check with a toothpick to be sure it is done.
Once cooled spread with frosting.