Preheat oven to 350 degrees F, line square 8" x 8" baking dish with unbleached parchment paper and spray with cooking spray.
In a large mixing bowl, add oats, flour, cane or coconut palm sugar (if using maple syrup add it later with liquids), baking powder, cinnamon and salt; mix well with spatula.
Add maple syrup (only if not using sugar), egg (quickly whisked in a small bowl), coconut oil and applesauce. Mix with a spatula and with your hands more towards the end until well incorporated. Dough will be moist and should stick to one piece, not crumbly.
In a medium bowl, add blueberries, maple syrup (sugar), orange zest, vanilla, cornstarch and stir well.
Place 3/4 dough in prepared baking dish, flatten with spatula and press well with your hands.
Spread blueberry filling on top evenly and crumble remaining dough on top. Make sure crumbs are small - will be easier to achieve with sugar dough, and crumb will be bigger with maple syrup dough.
Bake for 45 minutes. Remove from the oven and let cool completely before slicing into 16 squares.
Notes
Store: Store in a cool dry place for up to 3 days, refrigerate in an airtight container for up to 5 days.
Freeze: Freeze in an airtight container for up to 3 months.
Sweetener of choice: I have included 2 sweetener options: maple syrup or cane/coconut palm sugar. I tried both and I personally prefer the sugar version because bars and their crumb taste and look more like the original. With maple syrup, the bottom layer was a bit fluffier and cake-y but still delicious.
Fresh or frozen blueberries? I also prefer to use frozen blueberries. They pop during baking creating a nice singly thick layer of jam like filling. Fresh blueberries work as well but I found they don't pop as much during baking and stick out.
Flour: You can use only whole wheat or spelt flour. Maybe gluten free flour blend or white flour will work. Almond flour or coconut flours will not work.
Butter: I have not tested but I think it will work. Just make sure it is very soft so you can crumble it.
Other berries: Raspberries, blackberries, Saskatoon berries will work.