Preheat oven to 400°F. Butter 9x9x2-inch baking pan. Mix first 5 ingredients in medium bowl. Whisk buttermilk, oil, sugar and eggs in large bowl to blend. Add dry ingredients, stirring just until blended.
Transfer batter to prepared pan. Bake until tester inserted into center comes out clean, about 25 minutes. Cool in pan on rack. (Can be made 8 hours ahead. Cover; store at room temperature.)