Heat the oven to 350°F (175°C). Grease a 9-by-5-inch loaf pan and line it with parchment paper, leaving enough overhang on the long sides to lift out the finished loaf.

Whisk the flour, baking powder, baking soda, and salt together in a medium bowl. In a separate small bowl, stir the fresh lemon juice into the sour cream.

Beat the softened butter, granulated sugar, and lemon zest for 3–4 minutes, until pale and fluffy. Adding the zest to the sugar and butter releases the lemon oils throughout the batter.

Add the room-temperature eggs one at a time, beating well after each addition. Mix in the vanilla extract, scraping the bowl so the mixture stays even.

Add the dry ingredients and the sour cream mixture alternately in two or three additions, beginning and ending with the dry ingredients. Mix only until the batter is combined, since prolonged mixing can make the loaf dense.

Spread the batter evenly in the prepared pan. Bake for 50–60 minutes, until a skewer inserted in the center comes out with a few moist crumbs; loosely cover the top with foil during the final 15 minutes if it browns too quickly.

Cool the loaf in the pan for 10–15 minutes, then use the parchment overhang to lift it onto a wire rack. Cool completely before adding icing so the glaze remains thick and attractive.

For the optional syrup, simmer the sugar and lemon juice briefly, just until the sugar dissolves. Brush the syrup over the warm loaf, then let the loaf cool completely before icing.

Whisk the confectioners' sugar, 2–3 tablespoons of lemon juice, and the optional pinch of salt into a thick, pourable glaze. Drizzle it over the cooled loaf and let the icing set before slicing.