Preheat oven to 350.

Spray a jelly roll pan with non-stick cooking spray.

In large mixing bowl, combine flour, sugar, baking soda, and salt. Set aside.

In small mixing bowl, combine buttermilk, eggs, and vanilla. Set aside.

In saucepan on medium heat, add butter, cocoa, and water. Bring this mixture to a boil, stirring frequently.

Add saucepan mixture to the large bowl of dry mixture. Mix to combine.

Quickly add the wet ingredients and mix.

Pour mixture into jelly roll pan. Bake 15-20 minutes or until toothpick comes out clean.

Place on wire rack to cool and make the frosting.

Melt in a saucepan, butter, 4 tablespoons cocoa and 6 tablespoons buttermilk and stir to combine. Bring to a boil, stirring frequently.

Add confectioners sugar and vanilla and stir to combine.

Pour evenly over warm cake.

Let frosting cool, then cut and serve.