Place the chocolate and butter in a heatproof bowl and set over a saucepan of barely simmering water, making sure the base of the bowl does not touch the water. Stir constantly until melted and smooth.
With a hand-held electric mixer, whisk the sugar and eggs together for 2-3 minutes until pale and fluffy. Slowly add the melted chocolate and butter then add the vanilla extract and continue to whisk until thickened. Lastly, sift in the flour and baking powder and fold in gently.
Place the icing sugar and egg whites in a standing food mixer and whisk on high for 10 minutes until glossy white peaks form. Using a spatula, gently fold in the cornflour and the white wine vinegar. Pour the mixture on top of the brownie mix and bake for another 20 minutes, remove from the oven and allow to cool completely.
Once cooled, remove from the tin onto a cake stand. Whip the cream with the icing sugar until pillowy but still soft and then fold in the brandy. Dollop onto the top of the cake then sprinkle with the hazelnuts and gold stars and indoor sparklers and serve.