Preheat the oven to 350°. Grease & lightly flour 2 9-inch round cake pans or 1 9x13 baking dish. Set aside.
Heat the cup of water to boiling temperature.
In a large bowl, whisk together the flour, sugar, cocoa, baking powder, baking soda, salt, and espresso powder.
In another bowl, combine the milk, oil, eggs, and vanilla.
Add the wet ingredients to the dry ingredients and mix until well combined.
Carefully add the boiling water to the cake batter and beat on high speed for about 1 minute to add air to the batter. *The batter will be very thin after adding the boiling water. This is correct and is what makes it so moist.*
Distribute the cake batter evenly between the prepared cake pans and bake for 30-35 minutes or until a toothpick inserted in the middle comes out clean.
Remove from the oven and allow to cool in the pan for about 10 minutes then remove from the pan and cool completely.
Frost and serve.