Preheat oven to 350º.
Line 9x13 pan with nonstick foil.
In a large mixing bowl, whisk together flour, sugar, cocoa, baking soda, and salt.
Add butter, buttermilk and vanilla and beat for a couple minutes, scraping sides of bowl as needed.
Add eggs and beat for 2 more minutes, scraping sides of bowl often.
Pour into prepared pan and bake for 25-35 minutes or till center of cake bounces back when pressed with your finger. Cool pan on wire rack.