Method

Step 1
To make the genoise sponge: heat the oven to 180°C/160°C fan/350°F/Gas 4. Sift the flour and salt into a bowl and set aside. Melt the butter and leave to cool until needed. Break the eggs into a heatproof bowl, add the sugar and whisk with an electric hand whisk for a few seconds – just to combine. Set the bowl over a pan of simmering water (don't let the base of the bowl touch the water) and whisk for about 3–4 minutes until the mixture is very thick, mousse-like and tripled in size.

Step 2
Remove the bowl from the heat and whisk for another minute or so until the whisk leaves a distinct ribbon-like trail when lifted from the bowl. Sift the flour (a second time) into the bowl and carefully and gently fold into the mixture using a large metal spoon or plastic spatula. When ¾ combined, remove a couple of spoonfuls of the mixture and stir into the melted butter. Return this butter mixture to the bowl and continue to gently fold in until just thoroughly combined – check the base of the bowl for streaks but avoid over-mixing.

Step 3
Gently transfer the mixture to the prepared tin and spread evenly. Bake the sponge in the heated oven for about 16–18 minutes – until risen, golden brown and springy when gently pressed in the centre. Unclip the tin and turn out the sponge onto a wire rack. Leave to cool completely. Meanwhile, wash and dry the tin and fit the card cake base into the bottom of the tin. Cut a wide strip of acetate to fit all around the inside of the tin and secure with sticky tape. When the sponge is cool, return it to the tin, gently it pressing in place. If necessary, adjust the acetate collar so it fits snugly around the sponge.

Step 4
To make the coulis: put the raspberries, lemon juice, sugar and water into a food processor and blitz to make a thick puree. Pour into a medium-sized saucepan and bring to the boil. Cook gently (stirring frequently) for 3 minutes until slightly thicker, then remove from the heat. Sprinkle the gelatine over the mixture then stir for a minute until completely dissolved. Strain into a jug, pressing down on the pulpy mixture, to make a smooth puree (discard the seeds). Leave to cool to room temperature then chill for a few minutes until thickened and on the point of setting. Pour on top of the sponge and spread to make an even layer. Chill until set and firm.

Step 5
Now make the crème brûlée mixture: heat the milk with the cream and vanilla paste until steaming hot, then remove the pan from the heat. Put the egg yolks, cornflour and caster sugar into a heatproof bowl and beat well, using a wire hand whisk or electric hand whisk, until very light and smooth. When the mixture is very thick and light, whisk in the hot cream mixture (on a slow speed). Return the mixture to the pan and stir constantly over a medium heat until the custard just comes to the boil and is very thick and smooth. Pour into a heatproof bowl and press a piece of cling film onto the surface to stop a skin forming. Leave to cool to room temperature.

Step 6
Meanwhile, make the banana mixture: weigh the butter and sugar into a heavy-based frying pan and heat gently until melted and smooth, stirring frequently. Cut each banana in half lengthwise and add to the pan along with the syrup (if using). Leave to bubble away, fairly gently, for about 5 minutes (stirring occasionally) until the bananas are soft and the caramel sauce is thicker and sticky. Stir in the rum and simmer for another 4 minutes. Remove from the heat and mash roughly with a fork – leaving a few lumpy bits. Transfer to a heatproof bowl and leave to cool to room temperature.

Step 7
Give the crème brûlée mixture a good whisk, using a wire hand whisk, then spoon half into the tin, on top of the raspberry coulis, and spread evenly. Put into the freezer for about 20 minutes until firm, then add the cold banana mixture, spread evenly and return to the freezer for 30 minutes. Spread the remaining crème brûlée mixture evenly over the top of the banana layer and return the tin to the freezer for an hour.

Step 8
Now make the chocolate mousse: put the leaves of gelatine into a bowl and add cold water to cover. Leave to soak and soften while you make the custard: put both chocolates into a large heatproof bowl. Put 80ml of the cream and all the milk in a heavy-based, medium-sized pan and heat until steaming hot. Remove the pan from the heat. Whisk the caster sugar and egg yolks in a heatproof bowl until very light and smooth then whisk in the hot milk/cream mixture (on a slow speed). Return the mixture to the pan and stir over a medium-low heat until the custard thickens to coat the back of the spoon – don't let the mixture get too hot, or come anywhere near boiling, or it will curdle. Remove the pan from the heat.

Step 9
Drain the gelatine and squeeze out the excess water, then add the gelatine to the hot custard and stir until melted and smooth. Place a sieve over the bowl of chocolate and strain the custard/gelatine mixture into the bowl. Leave for a minute then stir gently until melted and smooth. Leave to cool to room temperature.

Step 10
Meanwhile, whisk the remaining cream until it stands in soft peaks when the whisk is lifted. Gently fold the cream into the cooled chocolate mixture. Spoon this chocolate mousse on top of the crème brûlée in the tin and spread evenly. Freeze for about 3 hours or until very firm, then transfer to the fridge – at this point, the dessert can be stored in a plastic container in the fridge for up to 24 hours.

Step 11
To finish: unclip the tin and peel away the acetate collar. Set the dessert, on its card base, on a serving platter. Decorate the top with fresh raspberries then scatter with the dark chocolate curls. If necessary, return to the fridge but remove 15 minutes before serving to allow it to 'come to'.