Preheat the oven to 350°F (176°C). Coat an 8"x8" inch pan with nonstick cooking spray.
Peel, core, and chop the apples.
In a large bowl, whisk together flours, baking soda, cinnamon, and salt.
In another large bowl, whisk together sugar, oil, yogurt, egg, and vanilla extract.
Add the egg mixture to the flour mixture and stir until just combined. Fold in the chopped apples. Scrape the batter into the prepared pan and spread it out evenly.
Bake for 35 to 40 minutes, or until a wooden toothpick inserted into the center comes out clean. Remove from the oven and cool in the pan on a wire rack. Cut into 16 even squares and serve warm or at room temperature.