Preheat oven to 350°F.
Grease the sheet pan well.
Mix flour, sugar, soda, and salt.
Heat butter, water, and cocoa together.
Bring mixture to a gentle boil.
Pour hot mixture over dry ingredients.
Stir until smooth.
Add buttermilk, eggs, and vanilla.
Mix gently until combined.
Pour batter into prepared pan.
Bake for 20 minutes.
Start frosting while cake bakes.
Melt butter with cocoa and milk.
Remove from heat.
Whisk in powdered sugar and vanilla.
Pour frosting over warm cake.
Spread evenly.
Let cake set before slicing.