1. Preheat your oven to 375 F (190 C). Lightly grease and flour a 9×13-inch baking pan, or line it with parchment paper, leaving an overhang on the sides for easy removal. This step is crucial for preventing sticking.

2. In a large mixing bowl, whisk together the 1 1/2 cups all-purpose flour, granulated sugar, baking powder, and salt until well combined. Make sure there are no lumps of baking powder.

3. In a separate medium bowl, whisk together the sourdough starter discard, milk, melted butter, egg, and vanilla extract until smooth. Ensure the egg is fully incorporated and the mixture is uniform.

4. Pour the wet ingredients into the dry ingredients. Stir gently with a spatula or wooden spoon until just combined. Be careful not to overmix; a few lumps are perfectly fine and will result in a tender bar. Overmixing can lead to tough bars.

5. In a small bowl, toss the blueberries (fresh or frozen) with the 2 tablespoons of all-purpose flour. This light coating helps prevent the blueberries from sinking to the bottom of the batter during baking.

6. Gently fold the floured blueberries into the batter until evenly distributed. Again, avoid overmixing.

7. Pour the batter into your prepared 9×13-inch baking pan and spread it evenly with a spatula. Bake for 25-30 minutes, or until a wooden skewer or toothpick inserted into the center comes out clean. The top should be golden brown.

8. Let the bars cool completely in the pan on a wire rack before slicing. Cooling completely allows them to set properly, making for cleaner cuts.