Preheat oven to 350°F (176°C). Grease an 8 x 8" metal baking pan (DO NOT USE A GLASS PAN, see notes*) or line with parchment paper and set to the side.
Melt 8 tablespoons butter in a microwave-safe bowl or in a saucepan on the stovetop. Add the melted butter, ¼ cup sourdough discard, and 1 egg to a large mixing bowl and whisk together until smooth.
Add 1 cup light brown sugar and 2 teaspoons vanilla extract and mix together until the sugar has fully dissolved.
In a separate bowl, combine 1 cup all-purpose flour, ½ teaspoon baking powder, and ½ teaspoon salt and whisk together.
Add the dry ingredients and ½ cup of chocolate chips to the wet ingredients and stir to combine. Spread the batter evenly into the baking dish. Top with ¼ chocolate chips.
Bake for 20-25 minutes. Don't overbake! The center will be slightly undercooked and jiggly, but it will continue to cook as it cools, so wait until it is completely cooled to cut in. This helps prevent the blondies from overcooking and yields a chewy, fudgy texture. The top of the blondies will be puffed up when they come out of the oven but will sink back down as they cool.
Once the blondies are completely cooled, cut them into 9 large or 12 small squares. For cleaner cutting, pop the blondies in the freezer for 15-20 minutes after cooling, and then cut them into squares.