Preheat the oven to 350°F. Prepare an 8x8-inch baking dish by lining it with parchment paper, allowing the paper to overhang on two sides for easier removal. Lightly coat the bottom and sides with nonstick spray. Set the pan aside.

In a large mixing bowl, combine the flour, granulated sugar, lemon zest, baking powder, and salt by whisking thoroughly.

In a separate bowl or large measuring cup, whisk together the melted butter, eggs, egg yolk, lemon juice, and vanilla extract until smooth.

Add the wet mixture to the dry ingredients and stir just until the batter is fully incorporated.

Pour the batter into the prepared pan and spread it into an even layer.

Bake for 18 to 20 minutes, or until the surface springs back gently when touched. Allow the bars to cool completely in the pan.

To prepare the glaze, mix the powdered sugar, lemon juice, and lemon zest in a small bowl until the consistency is smooth and even.

Once the bars have cooled entirely, spread the glaze over the top using an offset spatula or a knife, smoothing it into a thin layer.

Refrigerate for 1 to 2 hours to let the glaze firm up. Use the parchment overhang to lift the bars from the pan and cut into squares. Keep chilled until ready to serve.