Spray a large baking dish (9x13" works best) with non-stick cooking spray. Use a smaller pan for thicker bars.
In a large bowl, whisk together almond flour, baking powder, salt. Mix in the brown sugar, flax seed, water, and vanilla. Add the coconut oil or butter and mix by hand until mixture crumbles. Add just over half of the batter to the bottom of the pan and press down tightly (the rest will be for topping).
Combine the frozen strawberries, raspberries, blueberries, blackberries, sugar, and cornstarch until berries are well coated. Layer evenly on top of the crust in pan. Crumble the rest of the flour mixture on top of the berries.
Bake for 45-50 minutes or until the top is slightly browned. Remove and let cool on a wire rack or in the refrigerator before cutting.
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