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