Bring milk and cinnamon to a simmer in a medium saucepan over medium-low heat, whisking occasionally and making sure milk doesn’t boil, until cinnamon is floral and fragrant, about 10 minutes.
Whisk in chocolate, sugar, vanilla, salt, and cayenne pepper and cook, whisking frequently, until mixture is smooth and creamy and chocolate is melted, about 5 minutes.
Pour the mixture through a strainer to get all of the cinnamon pieces out.
Divide hot chocolate among mugs. Top with whipped cream and dust with cocoa powder.
