In a large pan, heat together the butter and dark chocolate until melted and smooth. Leave to cool.
Preheat the oven to 150°C or gas mark 2 and line a muffin tin with 12 cases.
In a large bowl, mix together the brown sugar, eggs, vanilla, plain flour, baking powder and cocoa powder. Add the chocolate mixture and stir well.
Pour the mixture into the tin, spreading the mixture evenly between the 12 muffin cases.
Bake for 45 minutes.
Leave to cool and dust with icing sugar to serve.