Whisk together flour, cocoa powder, baking powder, baking soda, brown sugar and salt.
Add remaining egg, oil, vanilla and milk and mix well until combined.
Spoon into 4 cupcake liners and bake at 180°C/350°F for 12-14 minutes until a toothpick inserted into the centre comes out clean.
Transfer the cupcakes to a cooling rack and while the cupcakes are cooling prepare the frosting.
Store in an airtight container at room temperature for 3 days.