Preheat oven to 350 degrees and grease a 9x5 inch loaf pan and set aside.
In a medium bowl whisk together flour, baking powder, baking soda, and salt.
In a large bowl combine zucchini, eggs, sugar, oil, melted butter, and vanilla and mix well. Stir in lemon juice, lemon zest, and poppyseeds.
Add dry ingredients to wet ingredients and mix until just combined. Pour batter into prepared loaf pan. Bake for 45-60 minutes until an inserted toothpick comes out clean ( a few crumbs are okay but no wet batter). Allow too cool on a wire cooling rack.
While bread is cooling, prepare the glaze by creaming together cream cheese, butter, vanilla, and lemon juice. Add powdered sugar and mix til combined. Add milk 2 tablespoons at a time, mixing after each addition, until glaze is a pourable consistency.
Drizzle glaze over bread. Slice and serve or store in airtight container up to 5 days.