Preheat your oven to 350°F. Grease a 9x5 loaf pan or line with parchment paper.
In a large bowl, whisk together the eggs, sourdough discard, melted oil, maple syrup, and vanilla extract.
Stir in the grated zucchini until combined.
In a separate bowl, whisk the dry ingredients together.
Gently fold the dry mixture into the wet, stirring just until combined. Don’t overmix.
Fold in any add-ins you’re using (nuts, chocolate, flaxseed).
Pour the batter into the prepared loaf pan. Smooth the top, and sprinkle with cinnamon sugar if desired.
Bake for 45–55 minutes, or until a toothpick inserted in the center comes out clean.
(For muffins: bake 20–25 minutes.)
Let cool in the pan for 10 minutes, then transfer to a wire rack to cool fully.