Spiced Vanilla Custards with Sweet Potato Streusel

Place 1 tablespoon water in a small bowl; sprinkle gelatin over and set aside. Combine cream, milk, cinnamon, star anise, and 1/4 cup sugar in a medium saucepan and scrape in vanilla seeds; add pod. Bring to a simmer over medium heat. Remove from heat, cover, and let steep 20 minutes. Uncover infused cream and return to a simmer. Whisk egg yolks with remaining 2 tablespoons sugar in a medium bowl. Whisking constantly, gradually add 1/2 cup warm cream mixture to egg yolks. Reduce heat to medium-low and whisk egg yolk mixture into remaining cream mixture. Cook, stirring constantly, until custard is thick enough to coat a wooden spoon, about 5 minutes. Add gelatin; cook, stirring until dissolved. Strain custard into a measuring cup. Pour into eight 2 ounces demitasse cups or ramekins. Chill until set, at least 4 hours. Do ahead: Custards can be made 2 days ahead. Cover and chill.