Preheat the oven to 300°F (150ºC).
In a small saucepan over medium heat, bring 1½ cups of heavy cream, muscovado sugar, and salt to a simmer, stirring until the sugar is completely dissolved. Remove the pan from heat.
In another medium saucepan over medium heat, bring 6 tablespoons water and Demerara sugar to a boil, stirring until the sugar has completely dissolved. Continue to simmer the mixture, stirring occasionally, until it is bubbly and golden brown, about 5 minutes. Remove the pan from heat then slowly add the cream mixture, whisking until combined.
In a large mixing bowl whisk together the egg yolks and vanilla. Slowly add the cream mixture in a steady stream, whisking until thoroughly combined. Pour the custard through a fine-mesh sieve into a medium mixing bowl. Skim off any foam with a spoon and discard.
Evenly divide the custard among the ramekins. Arrange the ramekins in a roasting pan and add boiling water until the ramekins are covered about half way up.
Bake in the hot water bath until the custards are set around the edges but not fully cooked through, about 40 minutes.
Transfer the ramekins to a rack and let cool. Refrigerate until chilled, about 2 hours. The custard will continue to set as it cools.
While the pots de crème cool, add the remaining cup of cream to a medium mixing bowl and whisk until the cream forms stiff peaks. Refrigerate until ready to serve.
Serve the pot de crème chilled. Top each with whipped cream and sprinkle sea salt over top.