Heat the oven to 350°F (177°C) and grease a 9x5-inch (or 8x4-inch) loaf pan. Coat the corners well so the warm loaf releases cleanly after its initial cooling time.

Whisk the flour, baking powder, lemon zest, and salt in a medium bowl. Break up any concentrated bits of zest so the citrus is evenly spread through the dry mixture.

Beat the room-temperature butter and sugar with a paddle attachment or electric mixer on medium-high speed for 4-6 minutes, scraping the bowl when needed. The mixture should become pale ivory, noticeably fluffier, and lighter in texture.

Reduce the mixer to low and add the eggs one at a time, then mix in the vanilla extract and lemon juice before beating briefly on medium-high. The batter should look smooth and cohesive rather than separated or curdled.

With the mixer on low, add one-third of the flour mixture, then half the buttermilk; repeat with another third of flour, the remaining buttermilk, and the final flour addition. Stop as soon as no dry streaks remain, because extended mixing makes the crumb firm and rubbery.

Spread the batter in the prepared pan and bake for 45-55 minutes. The top should be golden brown, a toothpick should emerge with only a couple of moist crumbs, and the center should offer gentle resistance when lightly pressed.

Cool the loaf in the pan for about 15 minutes while stirring together the lemon juice and powdered sugar for the syrup, then turn the loaf onto a wire rack. Brush the syrup over the warm cake so it absorbs into the crumb instead of sitting on the surface.

Let the loaf cool completely, then stir the powdered sugar, milk, and lemon juice for the icing, beginning with 1 Tbsp lemon juice and adding the remaining lemon juice as needed. Drizzle the thick icing over the cooled loaf and leave it undisturbed until the surface sets.