Combine buttermilk and oats; cover and refrigerate 8 hours.
Preheat oven to 375°.
Lightly spoon flours into a dry measuring cup; level with a knife. Combine flours, baking powder, baking soda, and salt, stirring with a whisk.
Place sugar and butter in a large bowl; beat with a mixer at medium speed until light and fluffy. Add vanilla and egg; beat until well blended. Stir in oat mixture; beat until well blended. Add flour mixture, beating just until moist.
Spoon batter into a 13 x 9-inch baking pan coated with cooking spray. Bake at 375° for 30 minutes or until a wooden pick inserted in center comes out clean. Cool 10 minutes in pan on a wire rack. Cut into squares. Garnish with powdered sugar, if desired.