Preheat the oven to 350°F (175°C) and line a 12-cup cupcake pan with liners.
In a bowl, mix together the milk and vinegar. Let it sit for 5 minutes, then add the sugar, oil, and vanilla extract. Stir until the sugar is almost dissolved.
In a separate bowl, sift together all the dry ingredients (baking powder, baking soda, cocoa powder, flour, and salt).
Gradually add the dry ingredients to the wet ingredients and mix until fully combined.
Fill each cupcake liner about 3/4 of the way with the batter.
Bake in the preheated oven for 13-15 minutes or until the cupcakes are firm to the touch and a toothpick inserted into the center comes out clean.