Preheat your oven to 350 degrees Fahrenheit (175 degrees Celsius). Grease a 9-inch by 13-inch rectangular cake pan with butter and line the bottom with parchment paper. Set aside.
In a large bowl, whisk together the sugar, flour, cocoa powder, baking soda, baking powder and salt.
Add the milk, Baileys, vegetable oil, eggs and vanilla to a large liquid measuring cup, whisking them together with a fork.
Whisk the wet ingredients into the dry ingredients just until the streaks of flour disappear.
Add the boiling water or hot coffee a little bit at a time, whisking until blended.
Pour the batter (it will be very liquid) into the prepared pan and baked at 350 degrees Fahrenheit (175 degrees Celsius) for about 30 minutes or until a toothpick inserted into the middle of the cake comes out clean.