Wash and dry the orange. Finely zest the whole orange, aiming for at least 2 teaspoons of zest, and set it aside. Then cut the orange in half and squeeze out at least 3 tablespoons of juice. Keep the zest and juice separate.

Pour the milk into a saucepan and add the orange zest. Set the pan over medium-high heat and bring the milk to a gentle simmer, stirring now and then. This should take about 5 minutes. Watch the pan closely so the milk does not boil over.

Add the semisweet chocolate to the warm milk. Stir until the pieces melt and the mixture looks smooth. Then remove the pan from the heat and stir in the pinch of salt. If you are making the adult version, add the optional Grand Marnier now and mix well.

While the milk heats, beat the heavy whipping cream with an electric mixer until soft peaks form. Add the sugar and 3 tablespoons of orange juice. Continue beating until the cream forms firm peaks that hold their shape. Stop as soon as it reaches this stage.

Divide the hot chocolate among 4 mugs. Spoon or pipe the orange whipped cream over each serving. If you like, finish with a little extra orange zest or chocolate curls. Serve right away while the drink is warm and the topping is fluffy.