Preheat oven to 350°. Beat butter at medium speed with a heavy-duty electric stand mixer until creamy. Gradually add granulated sugar and light brown sugar, and beat until light and fluffy. Add eggs, 1 at a time, beating just until blended after each addition. Add sweet potatoes and vanilla, beating just until blended.
Stir together flour and next 6 ingredients. Add to butter mixture alternately with buttermilk, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Spoon batter into 2 greased (with shortening) and floured 9-inch round, 2-inch-deep cake pans.
Bake at 350° for 35 to 40 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on a wire rack 10 minutes; remove from pans to wire rack, and cool completely (about 1 hour).