Combine ¾ cup cream, 1½ cups milk and 5 tablespoons sugar in a medium saucepan over medium-high heat. Bring to the boil. Meanwhile, place custard powder in a small bowl.
Gradually stir in remaining milk to make a smooth paste. Stir custard mixture into cream mixture. Stir over medium heat for 3 to 4 minutes or until mixture boils and thickens. Remove from heat. Cool for 10 minutes.
Arrange cake in the base of a 10 cup-capacity glass serving bowl. Drizzle with juice. Top with 1/2 the nectarine. Pour over custard. Refrigerate for 1 hour or until cold.
Using an electric mixer, beat mascarpone, vanilla, remaining sugar and remaining cream until soft peaks form. Spread over custard. Refrigerate for 1 hour.
Meanwhile, make Rosewater jelly Combine sugar and 1¼ cups cold water in a small saucepan over medium heat.
Stir for 4 to 5 minutes or until sugar dissolves. Sprinkle over gelatine. Stir for 3 to 4 minutes or until dissolved. Remove from heat. Stir in rosewater. Strain into a jug. Cool to room temperature.
Arrange remaining nectarine over mascarpone. Gently pour over cooled jelly. Refrigerate overnight, if time permits. Serve.