Special equipment: 6 (1/2-cup) decorative metal molds
Sprinkle gelatin over milk in a small bowl and let stand 1 minute to soften.
Whisk together confectioners' sugar, salt, and 1 cup of the heavy cream.
Cook granulated sugar in a dry small heavy saucepan over moderate heat, undisturbed, until it begins to melt. Continue to cook, stirring occasionally with a fork, until sugar is melted into a golden caramel. Stir in 1 1/2 teaspoons zest and cook, stirring, until zest is toasted and fragrant, 30 seconds to 1 minute. Stir the cream mixture and carefully add to caramel (it will bubble and harden). Cook over moderately low heat, stirring, until caramel is dissolved.
Stir in gelatin mixture and remaining teaspoon zest until gelatin is dissolved. Stir in orange juice, then let stand just until cooled to room temperature. Pour through a fine sieve into a medium bowl.
Beat remaining 1/2 cup heavy cream with an electric mixer until it just holds soft peaks. Whisk sour cream in another bowl until smooth. Fold whipped cream into sour cream, then fold into caramel mixture until combined well. Whisk to remove any lumps. The mixture should be smooth.
Spoon into molds and chill until firm, at least 8 hours.
Dip molds in hot water 2 or 3 seconds, until it begins to melt slightly on the edges and run a thin knife around edge of molds to help release panna cotta. Invert onto dessert plates and let stand at room temperature 20 minutes.
While panna cotta is standing, remove peel and white pith from oranges with a sharp knife, and cut the sections free from membranes.
In a small saucepan, combine the sugar, orange juice, and vanilla bean and bring to a boil. Reduce the mixture to 1 1/4 cups. Remove from the heat. If using the orange liqueur, stir it in.
Allow to cool before serving over the panna cotta.