Preheat the oven to 350°F (180°C). Line the bottom and two sides of a 9x9 baking pan with parchment paper for easy removal, then lightly grease the remaining sides and set aside. Dice the apples into small cubes and toss gently with 1/4 tsp cinnamon; keep them aside. In a medium bowl, combine the flour, cornstarch, baking soda, baking powder, salt, and 1 tsp cinnamon, stirring until evenly mixed, then set aside. In a separate large bowl, whisk together the oil, brown sugar, eggs, vanilla, and applesauce for 2–3 minutes, ensuring the sides of the bowl are well scraped. Add the dry mixture to the wet ingredients and fold gently with a spatula until just combined, taking care not to over-mix. Incorporate the chopped apples and fold briefly to distribute evenly. Spread the batter smoothly into the prepared pan. Mix the white sugar and 1/4 tsp cinnamon in a small cup, then sprinkle the mixture evenly over the batter for a sweet, crunchy topping. Bake for 25–35 minutes, or until a toothpick inserted comes out clean; the cake is typically done in about 30 minutes. Allow it to cool slightly, slice, and serve. Store leftovers in an airtight container at room temperature for up to 3 days or refrigerate for up to a week.