Preheat the oven to 180°C/fan 160°C/gas 4 and grease and base line 3 x 20cm shallow cake tins with non-stick baking parchment.
Beat together the butter and sugar in a large bowl with a hand held mixer until pale and creamy. Add the eggs a little at a time beating well between each addition, add the vanilla extract with the last of the egg.
Sieve together the flour, bicarbonate of soda and cocoa and add half to the bowl, fold in to mix, add the buttermilk, fold again then fold in the last of the flour mix. Add the cooled melted chocolate to the bowl and fold in until all fully incorporated.
Divide between the 3 lined cake tins and bake in the oven for 30 minutes until risen, browned and when a skewer inserted in the middle comes out clean. Leave to cool for 10 minutes in the tins before removing and leaving to cool completely on wire racks.
Meanwhile make the topping, heat the cream in a small pan, add 2 of the crème eggs, chopped and heat until melted and bubbling. Remove from the heat and whisk until smooth.
When ready to assemble the cake make the filling by whipping the cream until soft peaks form, scoop the flesh from the passion fruit and fold into the cream.
Place one of the sponges on a serving plate, spread with half the cream and top with a second sponge, spread over the remaining cream and top with the last sponge.
Cut the last 2 crème eggs into quarters and decorate the top. The cake can be kept in the fridge for a few hours, but take out about 30 minutes before serving.