Combine first 6 ingredients in a large bowl; make a well in center of mixture. Combine buttermilk, oil, and eggs; add to dry ingredients, stirring just until dry ingredients are moistened. Cover and chill at least 8 hours.
Preheat oven to 400°.
Spoon batter into 24 muffin cups coated with cooking spray, filling about three-fourths full. Bake at 400° for 14 to 15 minutes or until golden. Remove muffins from pans immediately, and serve warm.
Tip: You can make this muffin batter ahead and store it in the refrigerator up to three days.