Preheat the oven to 180°C/fan 160°C/gas mark 4. Grease and line a brownie tin with baking paper.
Break 150g of the chocolate into a heatproof bowl and place it over a pan of simmering water. Melt, stirring occasionally with a spatula, then stir in the softened butter until combined. Remove and set aside to cool slightly.
Whisk the sugar into the melted chocolate mixture. Slowly whisk in the eggs, then fold in the flour.
Pour the mixture into the prepared tin and bake for 20-25 minutes, until firm but still slightly squidgy. Allow to cool for 5 minutes in the tin, then remove and set on a wire cooling rack.
Meanwhile, heat the double cream until almost boiling, then turn off the heat. Break in the remaining chocolate and stir until combined. Allow to cool before spreading all over the brownies. Spray the top with gold glitter then sprinkle over the popping candy to serve.