Preheat oven to 350 degrees F. Liberally grease a 9x13" rectangular baking dish with cooking spray; set aside.

In a large bowl, combine the flour, sugars, baking soda, baking powder, nutmeg, cinnamon, and salt and whisk together until incorporated. Pour in the oil and stir to combine until moist crumbs come together. Reserve 2 cups of the crumbs.

Add in the beaten egg, buttermilk, and vanilla to the original bowl and stir until a thin batter comes together. Do NOT overmix the batter; some lumps are okay. Pour the batter into the prepared pan in an even layer. Sprinkle the top of the batter evenly with the crumbs.

Bake for 30-35 minutes or until a toothpick inserted near the center comes out clean or with moist crumbs. Cool about 15-20 minutes before cutting into squares to serve, or cool longer if you want prettier squares as this is a very tender cake!