In large mixing bowl, combine flour, sugar, baking soda, and salt. Set aside.
In saucepan on medium heat, add butter, cocoa, water. Bring this mixture to a boil, stirring frequently.
Add saucepan mixture to the large bowl of dry mixture. Mix to combine.
Add buttermilk, eggs, and vanilla and mix.
Pour mixture into un-greased jelly roll pan. Bake 15-20 minutes or until toothpick comes out clean.
For the frosting:
In a saucepan melt butter, cocoa and buttermilk and stir to combine. Bring to a boil, stirring frequently.
Remove from heat and add confectioners sugar and vanilla and stir to combine. Once smooth, stir in pecans if desired.
Pour evenly over warm cake.
Let frosting cool, then cut and serve.
Notes
If you don’t have buttermilk on hand you can make your own. Add 1 tbsp lemon juice or vinegar to 1 cup milk. Let it sit for a couple minutes and it’ll thicken right up.