Grease a 10x15 baking pan and preheat oven to 350 degrees.
Combine flour, sugar, baking soda, and salt in a large bowl. Whisk to incorporate and set aside.
Melt butter in a saucepan, whisk in cocoa powder and coffee until smooth, and bring to a boil. Remove from heat.
Combine buttermilk, eggs, and vanilla extract in a bowl and mix well. Add to the flour mixture and whisk for one minute.
Pour batter into the prepared pan and bake for 20-22 minutes. Prepare icing while the cake cools slightly.
Combine milk, butter, and salt in a saucepan, heat until simmering, then stir in vanilla. Sift powdered sugar and cocoa, then mix with the milk mixture until smooth. Pour over the warm cake and let set before serving.
Notes
Let the cake rest for a few hours before serving to allow the icing to set properly.