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 ingredients and set aside.
Melt butter in a saucepan, whisk in cocoa powder and coffee, and heat until boiling. Remove from heat.
Combine buttermilk, eggs, and vanilla extract in a small bowl. Mix well and set aside.
Pour the hot cocoa mixture into the flour mixture, stir to combine, then add the buttermilk mixture and whisk for one minute. Pour batter into the prepared pan and bake for 20-22 minutes.
Prepare icing by heating milk, butter, and salt until simmering. Stir in vanilla extract, then combine with powdered sugar and cocoa powder. Pour icing over the warm cake after 5 minutes.
Notes
Let the cake rest for a few hours before serving to allow the icing to set properly.