Zingy, citrus yuzu juice is used to good effect in this simplified showstopper. It infuses both the rich, white chocolate bavarois and the génoise sponge base. Impressive sugar shards complete the showstopper effect.
Heat the oven to 180°C/160°C fan/350°F/Gas 4.
To make the génoise sponge, melt the butter and set aside to cool. Break the eggs into a large heatproof bowl, add the yuzu juice and sugar, and whisk for 10 seconds using an electric hand whisk. Place the bowl over a pan of gently simmering water (don’t let the base of the bowl touch the water) and whisk on full speed for 7–10 minutes, until the mixture is mousse-like and the whisk leaves a ribbon-like trail when lifted.
Remove the bowl from the pan. Sift the flour and salt into the bowl, and fold in until almost combined. Stir 3 tablespoons of the mixture into the cooled melted butter, then return to the rest of the batter, fold in until combined, and pour, distributing evenly, into the prepared tin.
Bake the sponge for 22–25 minutes, until well-risen, golden brown and springy. Loosen the sponge from the edges of the tin using a round-bladed knife, then unclip the tin and turn out onto a wire rack to cool. Wash and dry the tin.
Make the bavarois. Soften the gelatine in water while you make the custard. Put the chocolate into a large heatproof bowl. Heat the milk in a medium pan, until steaming hot, then remove from the heat.
Meanwhile, put the yolks and sugar into another heatproof bowl and whisk with an electric hand whisk for 2–3 minutes, until thick and frothy. Whisk in the yuzu juice, then the hot milk.
Pour the mixture back into the pan and stir over a medium–low heat, until the custard thickens to coat the back of the spoon and leaves a path when you draw a finger through it – don’t let it boil. Remove from the heat.
Lift the gelatine from the soaking water, squeeze out the excess water, and stir the gelatine into the custard until melted. Strain the custard onto the chopped white chocolate and allow the chocolate to melt for 2 minutes, then stir until smooth and combined. Leave to cool to room temperature and start to thicken, stirring frequently.
Meanwhile, place the card cake base in the bottom of the clean springform tin. Place a wide strip of acetate around the inside of the tin to make a collar, and secure with sticky tape. Remove the baking paper, then place the sponge in the tin top downwards, pressing it in place.
Whip the cream for the bavarois until it forms soft peaks, then fold it into the custard until fully combined. Pour the bavarois onto the sponge and spread evenly. Chill for 3 hours, or overnight, to set.
To make the shards, put the sugar and 2 tablespoons of water into a small pan over a very low heat. Swirl the pan as the sugar dissolves, from time to time brushing down the sides with a heatproof pastry brush dipped in water.
As soon as the liquid is clear, turn up the heat so the liquid bubbles, and leave for 8–10 minutes, or until the bubbles get very large and cover the surface. Swirl, but don’t stir.
At the first hint of a colour change, remove the pan from the heat and pour the contents in a thin layer onto the oiled baking sheet. Sprinkle with the sesame seeds. Leave until set hard, then break into shards.
To assemble, place the dessert on a serving plate. Scatter the grated white chocolate or chocolate curls over the top, press the sesame shards upright into the bavarois, then serve with a bowl of fruit salad or berries, if you wish.