Preheat your oven to 375°F (190°C). Line a muffin tin with paper liners.
In a large bowl, whisk together the flour, cocoa powder, sugar, baking powder, baking soda, and salt.
In another bowl, mix the milk, vegetable oil, eggs, and vanilla extract.
Pour the wet ingredients into the dry ingredients and stir until just combined.
Divide the batter evenly among the muffin cups.
Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.