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.
Frosting
Sift icing sugar and cocoa powder into a small bowl with the softened butter and milk. Mix well until combined adding more milk if necessary until it is the right consistency for frosting.
Once the cupcakes have cooled completely, frost and decorate them with sprinkles or edible glitter.
Store in an airtight container at room temperature for 3 days.