Preheat the oven to 175 °C.
Grease a springform cake tin (22-26 cm in diameter).
Melt the butter, remove from the heat and add the chopped chocolate, stirring until it melts.
Whisk the eggs and sugar until fluffy.
Fold in the flour and add the melted chocolate.
Carefully mix in the smarties, marshmallows and nuts.
Pour the mixture into the cake tin.
Bake for 20-30 minutes.
Decorate with marshmallows and chocolate sauce.