Preheat your oven to 350°F (175°C) and grease a large baking sheet.
In a large bowl, combine flour, sugar, baking soda, and salt.
In a saucepan, melt butter with water and cocoa over medium heat, stirring until smooth.
Pour the butter mixture into the dry ingredients and mix until combined.
Add buttermilk, eggs, and vanilla to the batter, and mix well.
Pour the batter into the prepared baking sheet and spread evenly.
Bake for 20-25 minutes, or until a toothpick comes out clean.
While the cake is baking, prepare the frosting by melting butter with cocoa and milk.
In a separate bowl, mix powdered sugar with the melted cocoa mixture until smooth.
Once the cake is cool, spread the frosting evenly over the top.