Heat oven to 350℉. Butter and flour a loaf pan.
Using a microplane, zest the lemon. (For 1 tablespoon of zest, a single lemon will usually do.) Then juice the lemons, straining out the seeds, until you have ¼ cup lemon juice. (This will typically require about 2 or 3 lemons, depending on their size.)
Heat the milk until it is warm to the touch. Then sprinkle in the lemon zest to infuse. Set this mixture aside while you mix up the dry ingredients.
Melt the butter. Set aside to cool briefly.
In a large bowl, combine the flour, baking powder, salt, sugar, and poppy seeds.
In a medium bowl, combine the infused milk mixture, melted butter, sour cream, lemon juice, eggs, and vanilla extract. Whisk thoroughly.
Pour the wet ingredients into the dry, and then mix just until combined.
Pour the batter into the prepared loaf pan, smoothing the top.
Bake for about 70 to 75 minutes, or until the top of the cake is deeply golden brown, a tester inserted into the center comes out clean, and the sides have begun to pull away from the pan.