Preheat oven to 350°F (175°C). Line an 8x8-inch or 9x9-inch metal baking pan with parchment paper, leaving overhang, and lightly grease.
In a large bowl, whisk flour, oats, sugar, baking powder, and salt. Add cold cubed butter and vanilla; use a pastry cutter or fingers to blend into coarse crumbs with pea-sized butter bits for a perfect crumbly top.
Press two-thirds of the mixture (about 400-450g) firmly into the pan using a measuring cup to pack it evenly. Bake for 10 minutes to set.
Toss blueberries with sugar, cornstarch, lemon juice, and zest (if using) until coated. The lemon adds a bright, fresh note.
Spread blueberry filling over par-baked crust. Sprinkle remaining crumble loosely on top, letting some berries show through.
Bake 40-45 minutes, until topping is golden and filling bubbles. If using frozen berries, add 2-3 minutes.
Cool completely in the pan (2-3 hours). Lift out using parchment and cut into 12-16 bars.
Notes
Butter Tip: Keep butter cold and work it thoroughly for that gorgeous crumbly texture—don’t rush this step!
Berries: Fresh or frozen work; rinse frozen berries to remove ice, but don’t thaw.
Lemon: The lemon juice/zest enhances the blueberries subtly—no overpowering citrus here.
Potluck Win: These bars were a hit at my potluck, gone in minutes—make extra!
Storage: Store at room temp for 2 days, refrigerate for 5 days, or freeze for 3 months.
Make Ahead: Bake a day ahead; they hold up perfectly.