1. Preheat your oven to 350 F (175 C). Line a 9×13-inch baking pan with parchment paper, leaving an overhang on the sides to easily lift the bars out later. Lightly grease the parchment paper for extra non-stick insurance.

2. In a large mixing bowl, whisk together the melted butter, large egg, active sourdough starter, and vanilla extract until well combined and smooth. The mixture might look a little separated at first, but keep whisking until it’s uniform.

3. Pour the entire package of sugar cookie mix into the wet ingredient mixture. Use a sturdy spoon or a rubber spatula to mix until just combined. Be careful not to overmix; stop as soon as no dry streaks of flour remain. Overmixing can lead to tougher cookie bars.

4. Gently fold in the sprinkles until they are evenly distributed throughout the dough. Again, avoid overmixing, as excessive stirring can cause the colors to bleed.

5. Transfer the cookie dough to the prepared 9×13-inch baking pan. Use your hands or the back of a spatula to press the dough evenly into the bottom of the pan, making sure it reaches all the corners.

6. Place the pan in the preheated oven and bake for 20-25 minutes. The bars are done when the edges are lightly golden brown and the center is set. A toothpick inserted into the center should come out with moist crumbs, not wet batter.

7. Once baked, remove the pan from the oven and let the cookie bars cool completely in the pan on a wire rack. This is crucial for them to set properly and be easy to cut. Once fully cooled, use the parchment paper overhang to lift the entire slab of cookie bars out of the pan and cut them into squares or rectangles.