Preheat oven to 350°F and line the cupcake tray.
In a large bowl - whisk together the flour, sugar, cocoa powder, baking soda, and salt. In a medium bowl - whisk the buttermilk, oil, egg, and vanilla until well combined.
Pour the wet ingredients into the dry ingredients and whisk until well combined and no dry streaks of flour remain.
Divide the batter in the paper liners, filling about 2/3 full.
Bake for 18-20 minutes. (Let cool in baking tin for 5 minutes before taking out of cupcake tin).