1/2 cup (1 stick) chilled unsalted butter, cut into 1/2-inch pieces
Heat oven to 375 degrees F. Butter 9-inch square baking pan.
Pierce sweet potatoes in several places. Microwave on HIGH until tender, turning
once, about 12 minutes. Cut open and cool. Mash enough potatoes to yield 1 cup packed
(reserve remaining potatoes for another use). Place 1 cup mashed potatoes in large
bowl. Whisk in eggs and buttermilk.
Blend cornmeal and next 6 ingredients in processor. Add butter and blend until
mixture resembles coarse meal. Add to egg mixture. Stir just until blended. Transfer
to prepared pan.
Bake corn bread until deep golden on top and tester inserted into center comes
out clean, about 45 minutes.
Cool in pan on rack. (Can be made ahead. Cover and let stand at room temperature
up to 2 days or freeze up to 2 weeks. Thaw at room temperature.)