Preheat your oven to 350°F (175°C). Grease and flour an 8×4-inch loaf pan, or line with parchment paper for easy removal.

If using round pans, adjust baking time down slightly.

Whisk together the flour, baking powder, baking soda, and salt in a bowl. Sifting is optional but helps lighten the flour if your pantry is humid.

In a large bowl, beat the softened butter and granulated sugar until light and fluffy, about 3–4 minutes on medium speed. Scrape down the sides as needed.

Add eggs one at a time, beating briefly after each addition. Mix in vanilla and lemon zest. Stop mixing as soon as everything is combined.

With the mixer on low, add one-third of the dry ingredients, then half of the sour cream, then another third of the dry, then the remaining sour cream, and finish with the dry.

Mix just until combined. If batter seems too thick, stir in up to 1/4 cup whole milk.

Fold in the freshly squeezed lemon juice by hand. If adding poppy seeds, fold them in now.

Pour batter into prepared pan, smooth the top, and bake for 45–60 minutes for an 8×4 loaf, or 25–35 minutes for a 9-inch round.

A toothpick or cake tester should come out with a few moist crumbs, and an instant-read thermometer should read about 200–205°F in the center.

Allow the cake to cool in the pan for 10–15 minutes, then transfer to a cooling rack.

For the glaze, whisk sifted powdered sugar with lemon juice until pourable but not too thin. Drizzle over the warm or cooled cake.