In pan on stove, bring to a boil water, butter, cocoa powder, espresso powder and salt. Whisk to incorporate, remove from heat and set aside.
In the bowl of a stand mixer, combine dry ingredients and stir to combine.
Add dry ingredients to cocoa mixture and stir thoroughly to combine.
Add buttermilk, eggs and vanilla and mix until just combined.
Pour into greased and floured 9x13-inch cake pan. Bake 20 to 25 minutes at 400 degrees. While cake is baking, prepare icing.