Line the base of the springform pan with baking paper, grease the rim.
To melt the chocolate
Roughly chop both types of chocolate. Place each type in a thin-sided bowl. Suspend over a gently simmering bain-marie; the bowl must not touch the water. Melt the chocolate and stir until smooth.
Beat the butter in a bowl until soft. Whisk in the sugar, vanilla and salt. Add the eggs one at a time and continue to whisk until the mixture becomes lighter in colour. Divide the mixture in half, mix the white chocolate into one half and the dark chocolate into the other. Mix together the nuts, flour and baking powder, mix half into the white chocolate batter and the other half into the dark chocolate batter.
Spoon the light and dark batter mixtures into the prepared tin.
Approx. 1 h 5 mins. in the lower half of an oven preheated to 180°C. If necessary, cover with foil after 35 mins. Remove from the oven, allow to cool slightly, remove the cake from the tin and leave to cool on a rack.
Melt the cake icing in a bain-marie or in the microwave, pour over the cake, decorate with Smarties and sweets.