Preheat the oven to 350°F.

In a large bowl, beat granulated sugar, brown sugar, and softened butter with a mixer at medium speed until well blended and creamy.

Add egg whites and vanilla extract to the mixture and mix well.

In a separate bowl, lightly spoon whole wheat flour into a dry measuring cup, level with a knife.

Combine flour, graham cracker crumbs, baking powder, baking soda, and salt in the separate bowl.

Whisk the dry ingredients until well combined.

Gradually add the flour mixture to the sugar mixture alternating with buttermilk, beginning and ending with the flour mixture.

Spoon the batter into a 9-inch square baking dish that has been coated with cooking spray.

Tap the pan once on the counter to release any trapped air bubbles.

Bake in the preheated oven at 350°F for 35 minutes, or until a wooden pick inserted into the center of the cake comes out clean.

Remove the cake from the oven and let it cool in the pan for 10 minutes before removing it to a serving plate.

Cut into squares and serve. Enjoy!