Preheat the oven to 175°C and line your cupcake tray with 14 cupcake liners.
In a bowl, stir together the flour, baking powder, salt, and cocoa powder. In another bowl cream together the butter and sugar for about 3 minutes on medium-high speed.
Add the egg and beat for about a minute.
Combine together the buttermilk, vinegar, baking soda, and vanilla.
Add half of the flour and half of the buttermilk mixture to the butter mixture and mix just until combined.
Now add the other halves and briefly mix in until everything is well distributed and combined.
Distribute the cupcake batter to the prepared liners with an ice cream scooper or a spoon.
Bake for around 15-17 minutes or until an inserted toothpick or a clean knife in the center of the cupcake comes out with no wet batter.
Leave the cupcakes to cool completely.