Butterscotch Pots de Creme with Salted Caramel

Name: Butterscotch Pots de Creme with Salted Caramel
Ingredients: For the pots de crème:; 9 egg yolks, at room temperature; 1 1/4 cups packed dark brown sugar; 4 tablespoons unsalted butter; 3 1/2 cups heavy cream; 1 teaspoon kosher salt; 1/2 vanilla bean, split lengthwise; 1/4 cup granulated sugar; 2 1/4 teaspoons water; 1/4 teaspoon kosher salt; 1/3 cup heavy cream; 1/4 cup whipping cream; 1 tablespoon Buttermilk Crème Fraîche; Flaky sea salt for sprinkling; For the Buttermilk Crème Fraîche:; 4 cups heavy cream; 1 tablespoon buttermilk