01 - Preheat your oven to 350°F (175°C). Line an 8x8-inch (20x20 cm) baking pan with parchment paper, ensuring there's an overhang for easy removal of the finished bars.
02 - In a spacious mixing bowl, combine the all-purpose flour, rolled oats, light brown sugar, baking powder, and fine sea salt. Stir to ensure even distribution.
03 - Add the slightly cooled melted unsalted butter and pure vanilla extract to the dry ingredients. Mix until the composition forms moist, cohesive crumbs.
04 - Press approximately two-thirds of the prepared crumble mixture firmly into the bottom of the prepared baking pan, creating an even base layer.
05 - In a separate medium bowl, combine the pitted sour cherries with granulated sugar, cornstarch, fresh lemon juice, and finely grated lemon zest. Toss gently until the cherries are uniformly coated.
06 - Evenly distribute the dressed cherry filling over the pressed crumble base, spreading it gently to the pan's edges.
07 - Sprinkle the remaining oat crumble mixture evenly over the cherry layer, pressing down lightly to secure it.
08 - Bake for 35 to 40 minutes, or until the surface achieves a golden brown hue and the cherry filling appears to be bubbling.
09 - Allow the bars to cool completely in the pan situated on a wire rack. Once cooled, utilize the parchment overhang to lift the entire block from the pan. Subsequently, cut into individual bars.