1. Preheat your oven to 375 F (190 C). Lightly grease an 8×8-inch baking pan with butter or cooking spray, then line it with parchment paper, leaving an overhang on two sides. This parchment “sling” will make it easy to lift the bars out later.

2. In a large mixing bowl, whisk together the all-purpose flour, rolled oats, light brown sugar, baking soda, and salt until they are well combined. Make sure there are no lumps of brown sugar.

3. In a separate medium bowl, combine the melted butter, active sourdough starter, and vanilla extract. Whisk until smooth. The sourdough starter should be bubbly and active, indicating it’s ready to contribute its unique flavor and texture.

4. Pour the wet ingredient mixture into the dry ingredient mixture. Stir with a spoon or spatula until just combined. Be careful not to overmix; a few streaks of flour are fine. The dough will be thick and crumbly.

5. Gently fold in the fresh blueberries. Try to distribute them evenly throughout the dough without crushing them too much. If using frozen blueberries, add them directly from the freezer.

6. Transfer the dough mixture into your prepared 8×8-inch baking pan. Using the back of a spoon or your hands, press the mixture down firmly and evenly into the pan. Ensure it’s compact and reaches all corners.

7. Bake for 25-30 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out clean (or with just a few moist crumbs attached). The top should look set and slightly firm to the touch.

8. Remove the pan from the oven and let the bars cool completely in the pan on a wire rack. This is a crucial step! Cooling completely allows the bars to firm up, making them much easier to slice neatly without crumbling. Once cooled, use the parchment paper overhang to lift the entire slab out of the pan, then cut into desired bar sizes.