1. Preheat your oven to 375 F (190 C). Line an 8×8-inch square baking pan with parchment paper, leaving an overhang on two sides to easily lift the bars out later. Lightly grease the parchment paper.

2. In a large bowl, whisk together the gluten-free flour blend, rolled oats, granulated sugar, brown sugar, baking powder, and salt until well combined.

3. Add the cold, cubed dairy-free butter to the dry ingredients. Use a pastry blender, your fingertips, or a fork to cut the butter into the flour mixture until it resembles coarse crumbs with some pea-sized pieces remaining. Stir in the vanilla extract.

4. Press about two-thirds of the crumble mixture evenly into the bottom of the prepared baking pan. Use the back of a spoon or your hands to create a firm, compact base.

5. In a separate medium bowl, gently toss the blueberries (fresh or frozen, unthawed) with the lemon juice and cornstarch (or arrowroot powder) until the blueberries are lightly coated.

6. Spread the blueberry filling evenly over the pressed crumble base in the pan. Then, sprinkle the remaining one-third of the crumble mixture over the blueberry layer, creating the top crust.

7. Place the baking pan in the preheated oven and bake for 35-40 minutes, or until the top is golden brown and the blueberry filling is bubbly.

8. Remove the pan from the oven and place it on a wire rack to cool completely. The bars need to be fully cooled (at least 30-60 minutes, or even longer) before slicing to ensure they hold their shape. Once cooled, use the parchment paper overhang to lift the entire slab of bars out of the pan and cut into squares.