1. Preheat oven to 350°. Stir together first 4 ingredients and 2 Tbsp. water, and spread on bottom of a lightly greased 10-inch cast-iron skillet or a 9-inch round cake pan (with sides that are at least 2 inches high).
2. Beat granulated sugar and 1/2 cup softened butter at medium speed with an electric mixer until smooth. Add eggs, 1 at a time, beating until blended after each addition.
3. Combine flour, pumpkin pie spice, baking soda, and salt. Gradually add flour mixture to butter mixture, beating at low speed just until blended and stopping to scrape bowl as needed. Add sweet potatoes, buttermilk, and vanilla, beating at medium speed until smooth. Spoon batter over pecan mixture in skillet.
4. Bake on an aluminum foil-lined jelly-roll pan at 350° for 40 to 45 minutes or until a wooden pick inserted in center comes out clean. Cool in skillet on a wire rack 10 minutes. Carefully run a knife around edge of cake to loosen. Invert cake onto a serving plate, spooning any topping in skillet over cake.