1. Remove zest from orange in strips about 1-inch wide; set aside. In heavy 2-quart saucepan, whisk together cocoa, sugar, salt, and water over low heat until smooth. Simmer, whisking continuously, for 2 minutes, making sure whisk gets into the edges of pan.
2. Add milk and orange zest, increase heat to medium-low, and cook, stirring occasionally with whisk, until steam rises from surface and tiny bubbles form around edge, 12 to 15 minutes. Do not boil.
3. Add vanilla and half-and-half. Remove zest strips with a slotted spoon. For foamy cocoa, beat hot cocoa with hand mixer or transfer to blender and blend until foamy. Divide between four mugs, top with whipped cream or marshmallows if desired, and serve immediately.
STEP BY STEP: Dip and Sweep
1. To accurately measure flour, pour the flour into a large bowl, stir it with the measuring cup to aerate it, then dip the measuring cup completely into the flour.
2. Using a spatula, scrape all excess flour back into the bowl, leaving a precisely filled measuring cup.