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

2. In a large mixing bowl, whisk together the active sourdough starter, melted butter, light brown sugar, vanilla extract, and the large egg until the mixture is smooth and well combined.

3. In a separate medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Make sure there are no lumps and the ingredients are evenly distributed.

4. Gradually add the dry ingredient mixture to the wet ingredient mixture, stirring until just combined. Be careful not to overmix; a few streaks of flour are fine. Overmixing can lead to tough bars.

5. Gently fold in the rolled oats and the fresh or frozen blueberries. Stir just enough to distribute them evenly throughout the batter. If using frozen blueberries, do not thaw them first.

6. Pour the batter into your prepared 8×8 inch baking pan. Use a spatula to spread it evenly into the corners and flatten the top. The batter will be thick.

7. Bake for 25-30 minutes, or until the edges are golden brown and a wooden skewer or toothpick inserted into the center comes out clean. The bars should feel set to the touch.

8. Remove the pan from the oven and place it on a wire rack to cool completely. This is a crucial step; cutting warm bars will make them crumbly. Once fully cooled, use the parchment paper overhang to lift the bars out of the pan and slice them into squares or rectangles.