Preheat oven to 350°F.
Grease your cake pan lightly with butter, then sprinkle with flour to coat. Flip over to knock out loose excess (preferably over the sink or trash.)
In a mixing bowl, sift together the cocoa powder, sugar, salt, flour, and baking soda.
In a separate bowl, mix the vanilla and oil.
Mix the espresso powder into the boiling water until brewed as coffee. Then, add the coffee to the oil bowl.
Gradually add the coffee/oil to the dry ingredients, mixing until well combined.
Lightly whisk the eggs to mix them up, then stir them into the cake batter. The batter will be very thin, that's fine.
Divide your batter into your cake molds. I found using a makeshift piping bag to be easiest, but you can also just use a spoon.
Bake for 12-13 minutes, or until a toothpick or tester comes out clean. Remove from oven and let cool completely.