Preheat oven to 350°F and butter a 9x9-inch baking dish. Line the pan with parchment paper, extending on two sides for easy removal, and grease exposed parts.

In a small bowl, whisk together flour, baking powder, baking soda, and kosher salt.

In a large bowl, rub lemon zest into granulated sugar with your fingers until fragrant to activate the flavor.

Whisk melted butter until smooth, then whisk in eggs one at a time, followed by lemon extract. Switch to a spatula.

Add the dry ingredients and buttermilk, stirring gently until the batter is smooth. Spread the batter evenly into the prepared pan.

Bake for 30 to 35 minutes, or until the top is light golden brown and a toothpick inserted into the center comes out clean.

Cool the cake in the pan for 20 minutes. Use the parchment paper to remove the cake from the pan and let it cool completely to room temperature.

While the cake cools, mix powdered sugar with lemon juice, adding more if needed, until a thick but pourable glaze consistency is achieved.

Pour the glaze over the cooled cake and smooth with a spatula.

Cut the cake into pieces with a serrated knife before the glaze dries. Garnish with more lemon zest or slices if desired, and store in a sealed container at room temperature for up to 3 days or refrigerated for 1 week.