Heat the oven to 180C, 160C fan oven, gas mark 4. Grease and line the base and sides of a deep 20cm cake tin.
To make the cake, put 175g of the butter into the bowl of an electric mixer with the caster sugar, ½ the vanilla, eggs, flour and bicarbonate of soda. Beat together until very pale, light and fluffy.
Fold in the soured cream, orange zest and ground almonds.
Spoon half the mixture into the prepared tin. Stir Bonne Maman Mirabelle Plum (or Apricot or Peach) conserve into the sliced fruit and spread evenly over the cake mixture. Spoon on the remaining cake mixture and level the surface.
Bake for 40 minutes, cover loosely with foil and bake for a further 20 minutes or until a skewer inserted into the centre comes out clean. Leave to cool in the tin.
To make the frosting, beat together the remaining 50g butter with the remaining vanilla paste, mascarpone cheese and icing sugar until very smooth. Finally beat in the ricotta cheese. Keep chilled.
Cut the cake into 3 even layers. Put one sponge on a serving platter and spread with a layer of frosting using a palette knife. Top with a second sponge, spread again with frosting then finish with the top layer of sponge. Use about half the frosting to do this stage. Spread the remaining frosting over the top and sides of the cake, scraping here and there with the palette knife to give a mottled effect
Decorate with frosted rose petals, primroses and fresh rosemary.
To frost flowers and herbs, brush them very lightly with lightly whisked egg white then sprinkle with caster sugar. Leave to dry at room temperature.
Assemble cake without final coating of frosting the day before and keep chilled. About 4 hours before serving, finish with frosting and flowers. Keep in a cool place.
Beautifully sweet yellow Mirabelle Plums have a short season and are usually on sale in the UK late Summer. If not available sliced peaches or apricots also work well.
If you like an extra moist cake and want to use up the juice of the zested oranges then at point 7, cut the cake into 3 even layers and sprinkle each with the orange juice. This will also keep the cake moist if you wish to make the day before assembling and moving on to spreading each layer with the ricotta frosting.