Preheat oven to 350°F. Grease or line a 9×9" square baking pan and set aside.
Prepare cinnamon sugar: Mix together cinnamon and sugar in a small bowl and set aside.
Prepare streusel: Add coconut oil/vegan butter, brown sugar, oats, pecans, cinnamon, and salt (unless using salted butter) to a bowl. Use your hands or a fork to mix all the ingredients together until a thick mixture forms. Place in the fridge while you prepare the rest of the cake.
Prepare cake: In a large bowl, mix together soy milk, vinegar, applesauce, oil, and vanilla extract.
In a separate bowl, whisk together flours, sugar, baking powder, baking soda, and salt.
Add dry ingredients to wet and use a spatula to fold the ingredients together just until a uniform batter forms. Add chopped apples and gently fold into the batter.
Take the streusel out of the fridge. Add ½ the batter to the prepared baking dish. Sprinkle cinnamon sugar on top.
Add the remaining batter, then use your hands to sprinkle the streusel on top.
Bake 45-60 minutes until the center is set and a toothpick inserted in the center comes out clean. Place on a wire rack to cool. Let the cake cool completely before serving.