Preheat the Oven: Preheat your oven to 350°F (175°C) and grease a sheet pan.
Mix Dry Ingredients: In a large bowl, whisk together the flour, sugar, baking soda, and salt.
Prepare Wet Mixture: In a saucepan, bring the butter and water to a rolling boil. Remove from heat and whisk in cocoa powder. Allow it to cool for about 10 minutes.
Combine Mixtures: Add the cooled chocolate mixture to the dry ingredients. Mix until just combined.
Add Eggs and Buttermilk: Stir in the eggs, buttermilk, and vanilla extract until smooth.
Bake: Pour the batter into the prepared sheet pan, and bake for 20-25 minutes.
Make the Frosting: While the cake is baking, combine the butter, cocoa powder, and milk in a saucepan. Bring it to a boil, then remove from heat and stir in the powdered sugar and vanilla.
Frost the Cake: Once the cake is baked, let it cool slightly and then pour the frosting over the warm cake. Enjoy!