Follow the steps in order for the best result.
In a saucepan of medium size, blend the milk, brown sugar, and cracked cinnamon sticks. Heat this mixture until it's just about to boil. Then, take the saucepan off the heat, cover it, and allow the milk to steep and absorb flavors for approximately 20 minutes.
In a large mixing bowl, combine the egg yolks, granulated sugar, and vanilla extract. Whisk them together until the mixture becomes pale in color. Introduce the melted chocolate into this mixture and whisk until it's smooth and glossy, which should take about a minute. Note that the mixture will be quite thick at this point. Remove the cinnamon sticks from the infused warm milk and discard them. Gradually pour the warm milk into the chocolate mixture, whisking continuously to ensure a smooth and creamy consistency. Strain the resulting custard through a fine sieve to remove any lumps or impurities. Refrigerate the strained custard until it is thoroughly chilled, which will take at least 4 hours, or preferably overnight.
Get your oven ready by preheating it to 300°F (150°C). Prepare a water bath by placing a kitchen towel at the base of a large roasting pan. Arrange ten 4-ounce ramekins or espresso cups inside the roasting pan. Stir the chilled chocolate custard until it's smooth, then carefully divide it evenly among the ramekins. Pour hot water into the roasting pan so that it reaches halfway up the sides of the ramekins.
Cover the roasting pan with aluminum foil to create a sealed environment. Bake the custards for approximately 35 to 40 minutes, or until the edges are set, but the centers still have a slight jiggle. Carefully remove the ramekins from the water bath and allow them to cool to room temperature. Once cooled, refrigerate them for at least 3 hours to ensure they are thoroughly chilled. Before serving, garnish each pot de crème with a small dollop of lightly sweetened whipped cream and a sprinkle of ground cinnamon for an extra touch of flavor and presentation.