Preheat the oven to 350 degrees. Line a 9 X 13 pan with parchment paper and set aside.
Zest your lemons and squeeze out the juice, making sure to remove any seeds. I needed 2 lemons to get the right amount of zest and fresh juice because mine were smaller in size. Feel free to use bottled lemon juice to supplement if you don’t have enough for both the brownies and the glaze. Set aside both the zest and the juice in separate bowls or measuring cups.
Whisk together the flour and baking powder and set aside.
Using a stand or hand mixer, beat the butter, 2 tablespoons lemon juice, lemon zest and sugar until light and fluffy. Scrape down the sides of the bowl periodically.
Add the eggs one at a time and beat until combined.
Add the flour mixture gradually, scraping down the sides of the bowl as needed. Mix until just combined.
Spread the batter into the prepared pan evenly. Bake in a preheated oven for 25 to 28 minutes. Do not overbake. A toothpick inserted in the center should have some moist crumbs but no liquid batter.
Cool the brownies on the counter. While the brownies are cooling you can mix up the glaze.
For the glaze, mix the powdered sugar, lemon extract or vanilla, and lemon juice in a large measuring cup or medium bowl with a whisk until smooth. Start off with about half of the lemon juice and add more until you get the right consistency. You want a pourable glaze.
Once the brownies are cooled, pour the glaze over the top and spread to cover completely. Allow the glaze to harden and then cut the brownies.