Chocolate mousse charlotte

Heat the sugar and water in a small saucepan to 121 °C, checking with a thermometer. Meanwhile whisk the room-temperature whole egg in a stand mixer until pale. Lower the speed and pour the syrup in a thin stream down the bowl wall, avoiding the whisk. Whisk until cool, thick and foamy, with a clearly increased volume.

Melt the dark chocolate in a bowl over simmering water, without the bowl touching the water. Remove and let it cool slightly while remaining fluid. Whip the cold cream to soft peaks. When the chocolate and egg mixture are at similar temperatures, quickly stir the chocolate into the eggs, then gently fold in the whipped cream with a spatula.

Line the base of an 18 cm charlotte mould with baking paper. Mix the cane syrup and rum in a shallow bowl. Briefly dip the sponge fingers and arrange against the sides, rounded faces towards the mould. Fill with mousse, keeping any excess chilled in a piping bag for decoration. Trim the sponge disc to fit inside, lightly moisten with the rum mixture and place on the mousse; it becomes the base after unmoulding. Discard unabsorbed dipping liquid.

Cover the mould and refrigerate for about 720 minutes, overnight, until firmly set. Keep the reserved finishing mousse chilled too.

Invert onto a serving plate and remove the mould and paper. Pipe the reserved mousse into rosettes on top and scatter over the white chocolate chips. Keep chilled until serving.