1. Preheat oven to 350°. Beat butter at medium speed with an electric mixer until creamy. Gradually add sugar, beating well. Add eggs, 1 at a time, beating until blended after each addition.
2. Sift together flours, salt, and baking soda. Add flour mixture to butter mixture alternately with sour cream, beginning and ending with flour mixture. Beat at low speed until blended after each addition. Stir in vanilla.
3. Place peach slices in a lightly greased 12-inch cast-iron skillet; pour batter over peaches. Sprinkle with benne seeds.
4. Bake at 350º for 1 hour and 25 minutes to 1 hour and 35 minutes or until a wooden pick inserted in center comes out clean; let stand 10 minutes before serving.
Pantry Key. Benne seed: The Lowcountry cousin to the sesame seed, brought to the South from West Africa; used throughout the Sea Islands in desserts and breads. Bennecake flour: Benne seeds stone-milled into flour. Order from Anson Mills (ansonmills.com), which specializes in heirloom grains.