For extra texture, trick out your cake with rainbow or chocolate sprinkles, chopped candies, or your favorite cereal. Preheat oven to 350°. Lightly coat a 18x13" rimmed baking sheet with nonstick spray. Line with parchment paper; coat parchment with nonstick spray. Whisk flour, baking powder, baking soda, and salt in a large bowl until no lumps remain.