Melt 1/2 cup butter in a 10 1/2-inch cast-iron skillet over medium heat. Whisk in whipping cream and next 5 ingredients. Remove from heat.
Layer sweet potato slices evenly in cream mixture. Cover with aluminum foil, and place on a baking sheet.
Bake at 350° for 20 minutes. Uncover.
Cut 1/3 cup butter into flour with a fork or pastry blender until crumbly; stir in buttermilk until moistened. Turn dough out onto a lightly floured surface; knead 3 to 4 times. Pat or roll dough to a 10 1/2-inch circle; place over cobbler. Sprinkle with Streusel Topping.
Bake, uncovered, at 350° for 25 minutes or until golden.