For a richer flavor, use dark chocolate chips instead of white chocolate chips. If you don't have Demerara sugar, you can use turbinado sugar or coarse brown sugar as a substitute. The zucchini puree can be made ahead of time and stored in the refrigerator for up to 2 days. For an extra fudgy brownie, reduce the baking time by 2-3 minutes. Be sure to watch carefully to prevent burning. The walnuts can be toasted before chopping for enhanced flavor. Toast them in a dry pan over medium heat for 3-5 minutes, or in the oven at 350°F (175°C) for 5-7 minutes, until fragrant.