Preheat oven to 350°F. Grease a 9x5 inch loaf pan and line with a parchment sling.
In a large bowl, whisk flour, baking powder, baking soda, salt, poppy seeds, and lemon zest until evenly combined.
In a medium bowl, whisk sugar and eggs until slightly pale. Whisk in yogurt, milk, lemon juice, and vanilla. Slowly stream in melted butter, whisking until smooth.
Add wet ingredients to dry and fold gently with a spatula until just combined and no dry spots remain. Do not overmix.
Transfer batter to the prepared pan, smooth the top, and tap the pan once to release air bubbles.
Bake for 50 to 60 minutes, or until the top is golden and a toothpick inserted in the center comes out clean or with a few moist crumbs. If browning too quickly, tent loosely with foil for the last 10 minutes.
Cool in pan for 10 minutes, then lift out and cool completely on a wire rack.
Make the glaze: In a small bowl whisk powdered sugar with lemon juice and milk until smooth and pourable. Stir in optional zest.
Glaze the cooled loaf, let set for 10 minutes, then slice and serve.
Notes
For extra lemon flavor, rub the zest into the sugar before mixing. Store covered at room temperature for 2 days or refrigerate up to 5 days. Bread freezes well (without glaze) for up to 2 months.