Preheat the oven to 180ºc/Gas 4. Line 2 x 12 hole Yorkshire pudding tin with 12 cake cases (or bake in two batches). This will help to make perfect round cakes.
In a bowl add the flour, 50g cocoa powder and brown sugar.
In a large bowl combine the buttermilk, oil, eggs and vanilla extract. Pour all the dry ingredients into the buttermilk mixture and then carefully stir until all combined.
Spoon the batter into the cases and bake for 15mins, or until they spring back to the touch. Remove from the oven, allow to cool and then move to a wire rack and allow to cool. If you only have one 12 hole tin, repeat with the remaining cake mixture.
Add the icing sugar and 2 tbsp of cocoa powder in a small bowl and add 2 tbsp water and mix until smooth. Drizzle the frosting over the top of each cupcake, leave until the icing has set and serve.