Preheat oven to 350°F (175°C). Grease a 9x5 inch metal loaf pan thoroughly with butter, spray, or oil. For easy removal, line the bottom and long sides with a parchment strip, leaving overhang as handles.
In a medium bowl, whisk together the flour, baking soda, baking powder, salt, cinnamon, ginger, nutmeg, and cloves until evenly distributed. Set aside.
In a large bowl, whisk the granulated sugar, brown sugar, and oil together for about 30 seconds until combined. Add the eggs, pumpkin puree, sourdough discard, and vanilla. Whisk until smooth and uniform in color.
Pour the dry ingredients into the wet mixture. Use a spatula to fold gently with slow strokes, scraping from the bottom, just until no dry flour streaks remain. Do not overmix—a few lumps are fine.
Pour batter into the prepared pan and spread evenly. Optionally sprinkle 1 tablespoon of sugar over the top for a crunchy crust. Bake on the center rack for 55–65 minutes, or until a toothpick comes out clean or with a few moist crumbs.
Let the bread cool in the pan for 10 minutes, then use the parchment overhang to lift it out. Transfer to a wire rack and cool for at least 30 minutes before slicing to prevent a gummy crumb.
For the optional glaze, sift powdered sugar into a small bowl. Add milk and vanilla, then whisk until smooth. Drizzle over the cooled loaf and let set for 10 minutes before serving.