In a medium saucepan, add the cocoa power and 2 tsp of the sugar. Add boiling water and stir until dissolved. Reduce the heat to low and add milk, stirring regularly until the flavors combine. Sprinkle in cinnamon and stir to incorporate.
Pour 1 cup of hot chocolate into each of two mugs.
To make the whipped cream, add the Greek yogurt, whipping cream and the remaining 1 tsp of the sugar to a bowl. Using an handheld electric mixer or immersion blender with a whisk extension, beat the mixture until peaks form, about 3 minutes.
Top each cup of hot cocoa with ¼ cup of the whipped cream and sprinkle each hot chocolate evenly with the dark chocolate shavings. Serve warm.