Preheat your oven to 350°F (175°C).
Line your muffin tin with cupcake liners or lightly grease it.
In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt until well combined.
In a separate bowl, mix the milk, vegetable oil, egg, and vanilla extract. Whisk until well blended.
Pour the wet mixture into the bowl of dry ingredients. Stir gently until just combined.
Slowly add the boiling water to the batter and stir until smooth.
Fill each muffin cup about two-thirds full with the batter.
Bake for 18-20 minutes. Check for doneness with a toothpick.
Let the cupcakes cool in the tin for about 5 minutes, then transfer to a cooling rack.