Begin by zesting the orange, ensuring to avoid the bitter white pith. Gather a tablespoon of orange zest and combine it with 2 cups of whole milk in a medium saucepan. Heat over medium-low until warm, around 5 minutes.

Cut the orange in half and squeeze out the juice into a measuring cup, reserving 2 tablespoons for later. Add the squeezed orange halves and the zest-infused milk into the pot, covered.

Raise heat slightly to bring to a gentle simmer; once simmering, reduce heat to low and cover. Allow to steep for 10-15 minutes.

Carefully remove orange halves and zest. Shift heat back to medium-low and add in 4 ounces of dark chocolate, 2 tablespoons cocoa powder, 1 teaspoon cinnamon, and sugar. Whisk until smooth, around 3-5 minutes.

Stir in reserved orange juice and heat on low for another minute, whisking consistently to maintain a silky texture.

Pour hot chocolate into mugs, top with whipped cream if desired, sprinkle with cocoa powder, and finish with orange slices and zest. Serve immediately.