Preheat your oven to 350 degrees F. and line two baking sheets with parchment paper.
Mix the softened butter, 2 teaspoons of lemon zest, vanilla extract, and granulated sugar until well combined using an electric mixer. Mix for 3-4 minutes.
Add the egg and mix until well incorporated.
Add the flour, baking powder, and salt. Mix until well combined. About 1-2 minutes. Make sure to scrape the sides and bottom of the bowl.
Drop balls of cookie dough onto the baking sheets, making sure to space them 2 inches apart. Take each ball and roll it between your palms to shape it. Once shaped, use the palm of your hand to gently press down on the dough, flattening it to approximately 1/3 inch in thickness. Wet your hands before flattening them, this will ensure the cookie dough doesn't stick to your hands.
Bake for 12 minutes. While the cookies bake, prepare the lemon glaze. Simply combine the powdered sugar and lemon juice.
Once the cookies come out of the oven, allow them to cool for about 5 minutes before frosting them. Dip each cookie in the lemon glaze and garnish with the remaining lemon zest.
Allow the glaze to set for 10-12 minutes before serving!