Meanwhile, make the mousse
In a medium saucepan, combine 1/2 cup of the cream with the milk, zest and salt and bring to a simmer. While whisking, drizzle the hot milk mixture into the egg yolks until incorporated. Transfer the mixture to the saucepan and cook over low heat, stirring, until the custard is thickened and coats the back of a spoon, about 10 minutes. Remove from the heat. Add the gelatin and white chocolate and stir until melted. Strain the custard into a medium metal bowl set in a bowl of ice water and cool to room temperature, stirring, about 2 minutes. Remove the bowl from the ice bath.