Preheat your oven to 350°F (175°C) and line an 8x8-inch baking dish with parchment paper.

In a large mixing bowl, combine gluten-free rolled oats, baking powder, salt, chia seeds or ground flaxseed, and optional cinnamon.

In a separate bowl, whisk together unsweetened applesauce or mashed banana, pure maple syrup, melted coconut oil, and vanilla extract until smooth.

Pour the wet ingredients into the dry ingredients and mix until everything is evenly moistened.

Press about two-thirds of the oatmeal mixture firmly into the bottom of the lined baking dish.

In a bowl, toss mixed berries with maple syrup and cornstarch if desired, then spread evenly over the base layer.

Crumble the remaining oat mixture over the berry layer, pressing lightly.

Bake for 30-35 minutes or until the edges turn golden brown. Allow to cool completely before cutting into squares.