Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or grease it lightly.
In a large mixing bowl, combine the flour, sugar, cocoa powder, baking powder, baking soda, and salt. Whisk until well combined.
Add the milk, vegetable oil, egg, and vanilla extract to the dry ingredients. Mix until just combined.
Carefully stir in the boiling water until the batter is smooth. The batter will be thin.
Pour the batter into the prepared muffin tin, filling each cup about two-thirds full.
Bake for 18-20 minutes or until a toothpick inserted into the center comes out clean.
Allow the mini cakes to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.