1. Place all-purpose flour, whole wheat flour, brown sugar and butter in the KitchenAid® Stand Mixer bowl. Attach bowl and flat beater. Turn to STIR Speed and mix until butter is the size of peas, about 3 minutes. Stop and scrape bowl. Remove 1⁄2 cup flour mixture. Set aside.
2. Add baking powder, baking soda, and salt to flour mixture in bowl. Turn to STIR Speed and mix 30 seconds. Add buttermilk and egg. Continuing on Stir Speed, mix just until moistened, about 30 seconds. Do not overbeat.
3. Spoon batter into greased 9x13-inch baking pan. Drop blueberry filling by tablespoonfuls on top of batter; swirl into batter. Sprinkle top with nuts and reserved flour mixture. Bake at 350°F for 30 to 40 minutes or until light golden brown.
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