Preheat your oven to 350°F (175°C). Grease a sheet pan.
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 and stir to combine.
Add buttermilk, eggs, baking soda, vanilla extract, and salt. Mix well.
Pour the batter into the prepared sheet pan. Bake for 20 minutes or until a toothpick inserted into the center comes out clean.
While the cake is baking, prepare the frosting. In a saucepan, melt butter. 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.