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

2. In a large mixing bowl, whisk together the old-fashioned rolled oats, 1 cup all-purpose flour, granulated sugar, light brown sugar, baking powder, ground cinnamon, and salt until well combined. Ensure there are no lumps of brown sugar.

3. In a separate medium bowl, whisk together the melted unsalted butter, sourdough starter, and vanilla extract until smooth. The sourdough starter should be unfed and at room temperature for the best consistency.

4. Pour the wet ingredient mixture into the dry ingredient mixture. Use a fork or your fingertips to mix until a crumbly mixture forms. It should resemble coarse crumbs with some larger clumps.

5. Press about two-thirds of the crumb mixture evenly into the bottom of the prepared 8×8-inch baking dish. Use the back of a spoon or your hands to create a firm, compact base.

6. In a small bowl, gently toss the fresh blueberries with 2 tablespoons of all-purpose flour. This step helps prevent the blueberries from sinking to the bottom during baking. Sprinkle the floured blueberries evenly over the pressed oat base.

7. Crumble the remaining one-third of the oat mixture evenly over the blueberry layer. Don’t press it down; leave it loose for a nice crumbly topping.

8. Bake for 25-30 minutes, or until the top is golden brown and the edges are lightly crisp. A visual cue for doneness is when the blueberries are bubbly and the topping looks set.

9. Let the breakfast bars cool completely in the baking dish on a wire rack before attempting to slice them. This is crucial for clean cuts, as they will firm up as they cool.