1. Feed your sourdough starter discard until it is active and bubbly.

2. Preheat oven to 350°F (175°C) and line an 8×8-inch pan with parchment paper.

3. In a large bowl whisk together eggs, melted butter, milk, brown sugar, lemon zest, and vanilla.

4. In a separate bowl combine flour, salt, and baking soda.

5. Fold the dry ingredients into the wet mixture, alternating with the sourdough starter.

6. Toss blueberries in a teaspoon of flour to prevent sinking, then gently fold into the batter.

7. Pour batter into the prepared pan and smooth the top evenly.

8. Bake for 35 to 40 minutes until golden and a toothpick comes out with a few moist crumbs.

9. Let bars cool in the pan for 10 minutes, then transfer to a rack to cool completely before slicing.