Preheat oven to 300 °F. Line a cupcake tin with cupcake liners.
In a medium sized bowl, whisk together the buttermilk, egg, vegetable oil, and vanilla. Set aside.
In a large mixing bowl, combine flour, sugar, cocoa powder, baking soda, and salt. Whisk together.
Add the wet ingredients to the dry ingredients and mix until just combined.
Add the hot water (105 °F - 110 °F). Mix together for 30 seconds. Batter will be very thin.
Lastly, add the Baileys Irish Cream Liquor and mix for another 30 seconds.
Fill the cupcake liners about 3/4 of the way full. Bake for 22 minutes until a toothpick comes out clean.