Preheat the oven to 350°F (175°C).Line a 12-count cupcake pan with cupcake liners.
In a medium bowl, combine milk and lemon juice (or white vinegar). Let sit for 5-minutes to curdle slightly to form buttermilk.
Add sugar, oil, and vanilla to the milk mixture. Use a whisk to mix until the sugar is almost dissolved.
In a separate bowl, sift together the flour, cocoa powder, baking powder, baking soda, and salt. Gradually add the dry ingredients to the wet mixture. Use whisk to stir until smooth and lump-free.
Pour in the boiling coffee or boiling water and mix until fully incorporated using the whisk. Fill each cupcake liner about ¾ full with the batter using a ladle or ice cream scoop. You should get 12 cupcakes.
Bake for 13–15 minutes or until the tops are firm and a toothpick inserted comes out clean.