### Instructions
1. **Preheat & Prep:** Preheat your oven to 350°F (175°C). Line a 9×13 inch baking pan with parchment paper, leaving an overhang on the sides for easy lifting.
2. **Melt Butter:** In a large microwave-safe bowl or saucepan, melt the unsalted butter.
3. **Combine Wet Ingredients:** Add the granulated sugar and brown sugar to the melted butter. Whisk until well combined.
4. **Add Vanilla, Sourdough, and Eggs:** Stir in the vanilla extract, active sourdough starter, and eggs. Mix until everything is incorporated and smooth.
5. **Whisk Dry Ingredients:** In a separate medium bowl, whisk together the all-purpose flour, baking soda, and salt.
6. **Combine Wet and Dry:** Gradually add the dry ingredients to the wet ingredients, mixing on low speed or by hand until just combined. Be careful not to overmix.
7. **Fold in Chocolate Chips:** Gently fold in the chocolate chips until evenly distributed throughout the dough.
8. **Spread into Pan:** Press the cookie dough evenly into the prepared baking pan.
9. **Bake:** Bake for 25-30 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out with moist crumbs (not wet batter). The center may still look slightly underdone, but it will continue to set as it cools.
10. **Cool & Serve:** Let the cookie bars cool completely in the pan before lifting them out using the parchment paper overhang. Cut into squares and enjoy!
### Short Tips
* **Don’t overmix the dough:** Overmixing can lead to tough cookie bars. Stop mixing once the flour is just incorporated.
* **Use active sourdough starter:** A bubbly, active starter will give the best flavor and texture.
* **Vary your chocolate:** Feel free to experiment with different types of chocolate chips, chunks, or even add some chopped nuts for extra texture.
* **For extra gooeyness:** Slightly underbake the bars, but make sure they’re cooked enough to hold their shape.
* **Storage:** Store leftover cookie bars in an airtight container at room temperature for up to 3-4 days. They also freeze beautifully for longer storage!
