1 Heat the oven to 180°C (160°C in a fan oven), 375°F, gas 5. Brush the springform tin with butter and dust with flour.
2 Melt the chocolate in a bowl over a pan of hot water, then allow to cool slightly.
3 Beat the margarine with 150 g sugar until pale. Beat the egg yolks into the mixture and fold in the nuts. Stir in the melted chocolate. Mix the flour with the cocoa powder, sieve over the egg mixture and fold in.
4 Whisk the egg whites with the remaining sugar to form stiff peaks. Fold half into the egg mixture, then loosely fold in the remainder. Transfer immediately to the prepared tin, smooth the top flat and bake in the oven for around 1 hour (cocktail stick test). If necessary, cover with aluminium foil.
5 Remove from the oven and carefully release the edge of the springform tin. Cool the cake completely on a wire rack, then slice in half horizontally.
6 Warm the jam in a small pan until runny. Press through a sieve and stir in the rum. Spread half of the jam over the bottom half of the cake and put the top half on top. Spread the remaining jam around the sides of the cake.
7 Melt the plain chocolate with the coconut oil in a bowl over a pan of hot water, stirring. Cool slightly and then spread all over the cake.
8 Decorate the sides of the cake with chocolate stars as desired, and leave to dry. Before the icing is completely dry, garnish with writing (melted white chocolate) and sugar balls as desired.