1. Preheat oven to 350°.
2. Weigh or lightly spoon flours into dry measuring cups; level with a knife. Combine flours, sugar, baking powder, baking soda, and salt in a large bowl; make a well in center of mixture. Combine buttermilk, syrup, oil, and egg in a bowl; add to flour mixture, stirring just until moist. Stir in vanilla. Spoon batter into an 8 x 4–inch loaf pan coated with cooking spray. Sprinkle with walnuts. Bake at 350° for 50 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack; remove from pan. Cool completely on wire rack.