Preheat oven to 350°F (175°C). Grease a baking sheet.
In a mixing bowl, combine flour and sugar.
In a saucepan, melt butter. Add water and cocoa powder. Bring to a boil, then remove from heat.
Pour cocoa mixture over flour mixture and stir until combined.
Stir in buttermilk, eggs, baking soda, vanilla, and salt. Mix well.
Pour batter into prepared baking sheet. Bake for 20 minutes or until a toothpick comes out clean.
For the frosting, melt butter in a saucepan. Add cocoa powder and milk. Bring to a boil, then remove from heat.
Stir in powdered sugar and vanilla until smooth.
Pour frosting over warm cake. Let cool before serving.