Preheat the oven to 350°F (175°C). Line an 8x8-inch (20x20 cm) square baking pan with parchment paper, leaving an overhang for easy removal; lightly grease the parchment. In a large bowl, whisk together the all-purpose flour, rolled oats, brown sugar, granulated sugar, baking powder, and salt until evenly combined.

Cut in butter

Add the cold cubed unsalted butter to the dry mixture. Using a pastry cutter, two forks, or your fingertips, cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining. Stir in the vanilla extract so the mixture holds together slightly when squeezed.

Form base

Reserve about 1 cup of the oat-shortbread crumb mixture for the topping. Press the remaining crumb mixture evenly and firmly into the bottom of the prepared pan to form a compact base (use the flat bottom of a measuring cup or your fingers). Bake the crust in the preheated oven for 12–15 minutes, or until the edges are lightly golden and the base is set. Remove from the oven and set aside while you prepare the filling.

Date Filling

Soften and cook dates

Place the pitted dates and 1/2 cup water in a small saucepan over medium heat. Bring to a gentle simmer, stirring occasionally, and cook for 5–7 minutes until the dates are softened and the water is slightly reduced. If your dates are very dry, add an additional 1–2 tablespoons water as needed.

Make date paste

Transfer the softened dates and any cooking liquid to a food processor or blender. Add the brown sugar, lemon juice, vanilla extract, ground cinnamon, and 1/8 teaspoon salt. Process until smooth and spreadable, scraping down the sides as needed. The filling should be thick but spreadable; if it is too stiff, add up to 1 tablespoon more warm water and process again.

Assemble and bake

Spread the warm date paste evenly over the pre-baked oat-shortbread base, leaving the edges clean. Crumble the reserved oat-shortbread mixture evenly over the date layer to form the topping. Return the pan to the oven and bake for an additional 15–18 minutes, or until the topping is golden and the filling is bubbling slightly at the edges. Rotate the pan halfway through baking for even browning.

Cool and slice

Remove the pan from the oven and allow the date bars to cool completely in the pan on a wire rack (at least 1 hour) so the filling sets. Use the parchment overhang to lift the slab from the pan. Transfer to a cutting board and cut into 16 bars (4x4 grid) or 9 larger bars (3x3) as desired. Store in an airtight container at room temperature for up to 3 days or refrigerate for up to 7 days.