In a large heavy saucepan, bring cream just to a boil and remove from heat. Add chocolate and stir until it is smooth. Transfer mixture to a bowl and chill, covered, stirring occasionally, until it is cool and thickened. Do not let it solidify. Beat the mixture until it just forms soft peaks, but do not overbeat.
Force preserves through a fine sieve into a bowl.
Transfer the cake layers to a work surface. Trim edges and halve each layer lengthwise. On a serving plate, arrange one layer, spread with a third of the preserves, and spread a third of the ganache over the preserves. Top with another layer and proceed with the preserves, ganache and cake in the same manner, ending with a cake layer on top. Spread the sides and top of the cake with a thin layer of buttercream an chill for 30 minutes.