Marbled Souffles

Name: Marbled Souffles
Ingredients: Unsalted butter, room temperature, for ramekins; ½ cup granulated sugar, plus more for dusting; ¼ cup unbleached all-purpose flour; ¼ teaspoon kosher salt; 1 cup whole milk; 3 large egg yolks, plus 5 large whites, room temperature; 1 vanilla bean, split and seeds scraped; 2 ounces semisweet chocolate, coarsely chopped (½ cup), melted, and coole; 3 tablespoons sour cream; 2 large pinches cream of tartar; Confectioners' sugar and Dutch-process cocoa powder, for dusting; Vanilla ice cream, for serving (optional)