I seriously struggled to name this concoction. It was inspired by a wondrous dessert I was once served in a French restaurant – an individual half-sphere covered with white chocolate, hiding lovely raspberry mousse and delicate sponge inside. It came from a dessert trolley that had no labels, and it (rather stupidly) didn’t occur to me to ask what it was called.
I and my dining partner named it ‘The Bomba’. I think it was to mean ‘bubble’, rather than ‘bomb’. God knows why we resorted to Spanish, instead of calling it ‘bulle’ or ‘boule’. I think the ‘bomba’ seemed more resounding.
I went back home and tried to reproduce it, fighting with gelatine in the raspberries and trying various bowls for size to shape the sponge in. Not so difficult, once I decided to give up on gelatine and make the puree thickened with some corn flour at the second go. A lot of faff, admittedly, but it definitely delivers the ‘wow’ factor.
And I changed the name to ‘dome cake’.
1. The genoise can be, and in fact benefits from being made a day in advance – it will cut and handle much better when it’s sat in the fridge overnight. Preheat the oven to 180C/350F/gas 4. Prepare a round 20cm tin by buttering and flouring it thoroughly. Place a disk of parchment at the bottom. Mix the flours and sieve them 2 or 3 times.
2. Prepare a pan with hot (but not boiling) water – or bain-marie – large enough to contain the bowl you will be making the batter in. Break the eggs and the yolk into a large bowl or the bowl of a standing mixer. Place the bowl with eggs in the bain-marie and whisk for a few minutes adding the sugar little by little, with a hand whisk or a hand held mixer until the eggs come to about body temperature, digital probe or a cooking thermometer will be handy here. Make sure the eggs do not start to scramble!
3. Remove the bowl from the hot bath and beat in a standing mixer with a balloon attachment, or with a hand held mixer, at high speed for 5 minutes, adding the rest of the sugar if you didn’t manage it all at the initial stage. Reduce the speed to medium and give it further 5 minutes. The eggs should be five times the original volume, completely pale and quite stiff – or at least forming ribbons.
4. Sieve the flour over the egg mix from high above in two or three goes folding it in very gently each time before sieving in more. Make sure the mix does not deflate. Then get a couple of heaped spoonfuls of the batter and mix with the warm butter, then fold it gently back in.
5. Pour the batter into the tin and twist it sharply to remove large air bubbles. Bake for 25-30 minutes until the sponge crackles slightly when pressed with a finger and leaves a small indentation. Take out and drop from 20cm height onto a couple of folded tea towels, two or three times. This is the best part, and it works so well it’s amazing – contrary to appearance, it stops the sponge from collapsing and sinking. Turn the tin upside down onto a wooden board lined with a paper towel and leave for 5 minutes. Turn it the right side up, remove from the tin and cool completely on a cake rack. Wrap in cling film or a plastic bag and chill for at least a few hours.
6. To make the raspberry filling, place the frozen raspberries, water, sugar and lemon juice in a pan and bring to the boil. Simmer for about 15 minutes until the raspberries have broken down. Pass the mixture through a sieve to get rid of the pips and return to the pan (you might not bother if you think the seeds are not going to be a problem for anyone, in fact the filling sets better with seeds). Whisk in the corn flour mixture, stir in well and let it bubble for another 5 minutes until it thickens considerably. Transfer it to a bowl and cool completely. Chill in the fridge for at least a couple of hours and best overnight.
7. To make the dark chocolate mousse, break the chocolate into small pieces into a bowl and add the icing sugar. In a small pan or in a microwave bring the double cream with the butter to the boil and pour it boiling over the chocolate. Leave to stand for a couple of minutes and then stir it well. Leave to cool completely, then place in the fridge for at least half an hour.
8. Whip the chilled chocolate mix with a hand whisk or an electric mixer until it thickens, lightens in colour and has a mousse consistency – don’t over-whip. If that happens, add a spoonful of cream to the mousse and gently whip it in.
9. To make the buttercream, beat the butter until pale and fluffy. Still beating, add the icing sugar by a couple of spoonfuls. Add the scraped vanilla seeds and the cream, and whisk it in.
10. To assemble the cake, line a 20-23cm bowl with cling film. Slice the genoise horizontally into three layers using a cake slicing wire, a very sharp knife or a cotton thread (make a shallow incision with a sharp knife along the edge, halfway through its height. Take a length of cotton thread and wrap around the cake, fitting it into the incision. Twist the thread as if you wanted to strangle the cake until it cuts through). The middle layer should be really thin, as it’s to go between raspberry and buttercream inside the dome, but it’s optional.
11. Place one layer, cut side up, into the bowl lined with cling film, pressing gently in. Spread a thin layer of the dark chocolate mousse over it (you might want to take the cake out of the bowl for this exercise and return it again). Chill for half an hour to allow it to set.
12. Next, spoon in the raspberry filling – as much or as little as you like, mindful that there should be space left for the buttercream. Place the thin middle genoise layer, trimmed to the right diameter, on top of the raspberry filling, if you wish.
13. Pipe or spoon the buttercream on top of the previous layer, spreading some onto the edge as well. Finally, place the bottom cake layer to close the dome, cut side in. Press it gently in the middle, to spread the buttercream, and all around the edge to make sure it sticks. Chill in the bowl for an hour or overnight.
14. For the final decoration, turn the cake out of the bowl onto a serving tray or cake stand. To make the white chocolate ganache, break up the chocolate into pieces as small as possible. Put the cream and butter in a bowl and microwave for 40 seconds until it boils vigorously. Immediately add the chocolate and leave to stand (in the microwave) for a minute or two. Whisk together into smooth ganache. This will be fairly thick ganache so needs to be spread with a palette knife over the dome cake, rather than poured over. If you’d rather have a thinner, runnier ganache, increase the cream amount up to 80g.
15. Make sure the sides of the cake are covered with ganache, then throw some rose petals over it or decorate according to your taste. Chill, trim the edge of the ganache pooled at the bottom of the cake and transfer to a clean serving plate.