Preheat oven to 350°F (175°C) and grease/flour a sheet cake pan.

In a large bowl, whisk together the flour, sugar, baking soda, and salt.

In a saucepan, combine the butter, water, and cocoa powder. Bring to a gentle boil.

Pour the hot chocolate-butter mixture into the dry ingredients and stir until mostly combined.

Add in the eggs, buttermilk, and vanilla, stirring until the mixture is smooth.

Pour the batter into the prepared pan and spread evenly.

Bake for 20–25 minutes, or until a toothpick inserted comes out with a few moist crumbs.

While the cake is baking, melt the butter for the frosting in a saucepan, whisk in cocoa powder, then incorporate powdered sugar and milk until smooth.

Immediately pour frosting over hot cake and spread lightly.

Let the cake cool so the frosting sets before slicing.