Begin by preheating the oven to 350 degrees Fahrenheit. Prepare an 8X8-inch aluminum baking pan by lining it with parchment paper, ensuring it overhangs on two sides for ease of removal post-baking. Alternatively, you may grease the pan thoroughly.

Take a large mixing bowl and meticulously combine the all-purpose flour, granulated sugar, salt, baking powder, and lemon zest.

In a separate mixing bowl or a sizable liquid measuring cup, blend the cooled melted butter, two whole eggs, one egg yolk, fresh lemon juice, and pure vanilla extract.

Gradually incorporate the wet ingredient mixture into the flour mixture, stirring consistently to ensure a smooth, lump-free batter.

Transfer the batter into the pre-prepared baking pan, ensuring it is spread uniformly.

Place the baking pan in the oven and bake for a duration of 18-20 minutes. It's imperative to ensure that a toothpick, when inserted into the center of the baked batter, emerges clean or with only a few moist crumbs. Overbaking can compromise the desired moist texture of the lemonies.

Upon baking completion, allow the lemonies to cool.

For the lemon glaze, employ a separate mixing bowl and whisk the powdered sugar, fresh lemon juice, and lemon zest together until a smooth consistency is achieved.

Once the lemonies have sufficiently cooled, pour the glaze over them, ensuring even coverage. It is advisable to distribute the glaze uniformly, rather than concentrating it in one spot.

The glazed lemonies should be refrigerated for 1-2 hours, ensuring the glaze sets firmly. Afterward, employ the parchment overhangs to lift them out of the pan. They can be neatly sliced into squares and are best enjoyed when chilled. However, they can also be served at room temperature if desired.