Preheat oven to 350°F. Grease a 9 x 5 inch loaf pan and line with a parchment sling; lightly grease the parchment.
Whisk flour, baking powder, salt, poppy seeds, and lemon zest in a large bowl until evenly combined.
In a separate bowl, whisk sugar and eggs until slightly pale, about 0.5 minute. Whisk in milk, sour cream, melted butter, lemon juice, and vanilla until smooth.
Add dry ingredients to wet. Fold gently with a spatula just until no dry streaks remain; do not overmix.
Transfer batter to the prepared pan and smooth the top. Bake 50 to 60 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 using the parchment and cool on a rack until just warm or room temperature.
Make the glaze: Whisk powdered sugar with lemon juice, adding milk as needed to reach a thick, pourable consistency.
Pour glaze over the loaf, letting it drip down the sides. Sprinkle with lemon zest if using. Let set 10 minutes, then slice and serve.
Notes
For extra lemon punch, replace 0.25 cup of the milk with additional fresh lemon juice. Bread keeps covered at room temperature for 2 days or refrigerated up to 5 days. Freeze sliced, tightly wrapped, for up to 2 months.