Preheat oven to 325 degrees F. Grease and flour two 8 x 4 inch loaf pans and set aside.: The kitchen will warm and the faint smell of heat will signal readiness, which matters because a cold oven can ruin the loaf's rise. Use a light oil spray and dust with flour so the crust releases easily, you should hear a slight sizzle when the pans first hit the heat if preheating on a stovetop oven indicator, but mainly you want the pans to feel warm to the touch before batter goes in. This step ensures even baking and a uniform crust. One common mistake is skipping the flour after greasing which can cause sticking; take a moment to do both.
In a large bowl, whisk together the flour, salt, baking powder, and baking soda. Set aside.: You'll notice the dry mixture looks airy and uniform, and whisking incorporates the leaveners evenly so you get a consistent lift during baking. The combination of baking powder and baking soda gives balanced rise and fine crumb. If you skip thorough whisking, pockets of leavening can create uneven texture, so make sure the mixture looks homogenous and there are no lumps.
In a large bowl, combine sugar and lemon zest. Rub together with your fingers until fragrant.: Right away you'll smell intense citrus oils as the zest melds with the sugar, releasing a bright, perfumed aroma that quietly perfumes the whole batter. This fragrant step infuses the loaf with lemon essence without adding more liquid, which keeps the batter body intact. Be careful not to include the bitter white pith while zesting, which can give an unpleasant edge to the flavor.
Add the eggs, olive oil, lemon juice, vanilla to the lemon sugar mixture and stir until smooth. Add in the dry ingredients and stir until just combined, the batter will be thick. Stir in the zucchini and pour the batter into the prepared loaf pans.: As you stir, the mixture will become glossy and slightly ribboned because of the oil and eggs, and that sheen indicates the batter is well emulsified. The dry ingredients will disappear into the wet, forming a thick batter; you want minimal strokes to avoid overdeveloping gluten, which keeps the crumb tender. When folding in zucchini , watch for even distribution of green flecks; the batter should remain lumpy but cohesive. A frequent misstep is overmixing after adding flour, which causes a tough loaf, so stop as soon as you no longer see streaks of flour.
Bake for 60-65 minutes, or until tester inserted in the center comes out clean.: During baking you will see the tops turn a warm golden brown and the aroma will shift from citrus to toasted sweetness. The loaves will rise with a gentle dome, and a toothpick or cake tester should emerge clean or with a few moist crumbs. If the tops brown too quickly, tent with foil for the last portion of baking. Pulling the loaves too early will result in a gummy center, so rely on the tester rather than clock time alone.
Place the loaves on a cooling rack and let cool for 15 minutes in the pans. Loosen the sides of the bread with a knife. Carefully remove the loaves from the pans. Let cool completely on rack.: As they cool, the crumb firms and the internal steam settles, so this resting period prevents collapse. The loaves will give a soft sigh as they release from the pan when cooled slightly; use a thin knife around the edges to free them gently. A common error is removing loaves immediately, which can cause them to break apart, so be patient and let them rest.
While the bread is cooling, make the lemon glaze. In a small bowl, combine powdered sugar and lemon juice. Whisk until smooth. Drizzle the glaze over the loaves. Slice and serve.: The glaze should be glossy and pourable, with a scent of bright citrus that complements the baked loaf. Drizzling when loaves are still slightly warm helps the glaze set into the nooks of the crust without melting away. If the glaze is too thick, add a little more lemon juice ; if too thin, add a touch more powdered sugar. Drizzle slowly to control coverage; pouring too quickly will result in runoff and uneven coating.