Heat the saucepan over low-medium heat while stirring for 5 minutes. For best results, use a molinillo or whisk.
While the hot chocolate is cooking, prepare the whipped cream. Gradually increasing the speed, beat cream with powdered sugar until peaks form, about 3-4 minutes.
Remove hot chocolate from heat and pour it into mugs.
Top each mug with whipped cream.
If desired, sprinkle ¼ teaspoon cinnamon onto each cup.
Top each cup with a small handful of mini marshmallows and flaky salt if desired, then serve.