Preheat oven to 350°F. Grease a 8.5×4.5-inch loaf pan and line with a parchment sling.
Whisk flour, sugar, baking powder, baking soda, salt, poppy seeds, and lemon zest in a large bowl.
In a separate bowl, whisk eggs, sour cream, milk, lemon juice, and vanilla until smooth.
Slowly whisk in the melted butter until fully combined.
Make a well in the dry ingredients and pour in the wet mixture.
Fold gently with a spatula just until no dry streaks remain. Do not overmix.
Let the batter rest for 5 minutes to hydrate the poppy seeds.
Scrape batter into the pan, smooth the top, and tap the pan to release bubbles.
Bake 50–60 minutes until golden and a tester comes out clean. Cool 10 minutes in the pan, then finish cooling on a rack before slicing.