Preheat oven to 350°F. Grease a 9x5 inch loaf pan and line with parchment, leaving overhang for easy removal.
In a large bowl, whisk flour, baking powder, baking soda, salt, poppy seeds, and lemon zest.
In a medium bowl, whisk buttermilk, melted butter (cooled), lemon juice, eggs, and vanilla until smooth.
Add wet ingredients to dry and stir gently until just combined; do not overmix.
Transfer batter to the prepared pan and smooth the top.
Bake 50 to 55 minutes, until a toothpick inserted in the center comes out clean or with a few moist crumbs.
Cool in pan 10 minutes, then lift out and cool on a rack until warm.
For the glaze, whisk confectioners' sugar with lemon juice and milk until smooth and pourable.
Drizzle glaze over the warm loaf, sprinkle with poppy seeds if desired, and let set before slicing.
Notes
For extra lemon flavor, prick the warm loaf and brush with a quick syrup made from 2.0 tablespoons lemon juice and 2.0 tablespoons sugar before glazing. Store airtight at room temperature up to 3 days.