1. Preheat the oven to 180C/350F/gas 4. Butter two 20cm round cake tins and line their bottom with parchment.
2. First prepare the ganache for the whipped chocolate cream as it will need to chill in the fridge for as long as possible; it can easily be prepared on the previous day. Bring 150ml of the cream with the tablespoon of sugar almost to a boiling point. Pour it over the chocolate chips in a large bowl. Stir to melt and chill the ganache in the fridge.
3. For the cake, sift the flour, bicarbonate of soda, cocoa and sugar together into a large bowl; don’t skip this step, as cocoa tends to lump.
4. In a smaller bowl stir together the oil, vanilla and boiling water.
5. Gradually whisk the oil mixture into the dry ingredients until smooth. Whisk in the eggs; the batter will be very runny.
6. Divide it between the prepared tins, tap the tins against the worktop to remove air bubbles and transfer to the oven, on the middle rack. Bake for 25-30 minutes until a skewer inserted in the middle of the cakes comes out clean. Cool in the tins.
7. While the cakes are cooling make the buttercream. Scoop the flesh out of halved passion fruit and pass it through a sieve or a food mill with medium sieve, to get rid of the pips.
8. Beat the butter with about three quarters of the icing sugar starting from low speed, turning up to high, for about 5 minutes until light and fluffy. Beat on low speed gradually adding the passion fruit juice, then add the milk. Add the remaining icing sugar and beat briefly on high speed until the buttercream is smooth and fluffy.
9. To make the chocolate cream frosting, stir the remaining cream into the ganache – make sure both are really cold – and whisk by hand or with a handheld mixer to soft peaks, don’t overbeat.
10. To assemble the cake, turn the bases out of the tins and peel off the parchment. Spread the buttercream over the cake base and invert the other onto the buttercream; peel the other parchment off.
11. Pile the chocolate cream over the top of the cake and spread it with an offset spatula, pushing the cream onto the sides of the cake as well. Smooth the top with a bench scraper if you have one or a clean palette knife.
12. To decorate, grind the rose petals with icing sugar in a pestle and mortar (or add food colouring of your choice to the sugar). Sift the pink sugar and/or cocoa powder over the cake through a stencil.