Preheat oven to 350 degrees and spray an 8x8 pan with baking spray.
Mix brown sugar, walnuts and cinnamon in a small bowl and set aside.
To your stand mixer add the sugar, butter and applesauce until well combined on medium-high speed.
Add in the egg whites until creamy, then lower the speed to medium and add in the sour cream, buttermilk and vanilla until smooth.
Sift together the whole wheat pastry flour, oat flour, baking powder, baking soda and salt.
On the lowest speed setting add in the flour until just combined.
Add in half the batter to your baking pan, add half the walnut mixture to the batter, then spoon the remaining batter over the walnuts.
Top with the remaining walnut mixture and bake for 30 minutes until a toothpick comes out clean.