Preheat the oven to 350°F. Line two baking sheets with parchment paper.

Whisk flour, baking powder, and salt in a small bowl. Set aside.

In a large bowl, use an electric mixer fitted with the paddle attachment to beat together the butter, sugar, and lemon zest until light and fluffy, about 3 to 4 minutes, scraping down the bowl at least once.

Beat in egg until very well combined, about 1 minute. Scrape down the bowl.

On low speed, add the flour mixture and blend until just incorporated. Dough will be slightly dry.

Using a medium spring-loaded cookie scoop, drop 1 ½-tablespoon sized balls of cookie dough onto prepared baking sheets, spacing 2 inches apart. Roll each ball with your palms, then using your palm, gently flatten dough to about 1/3-inch thick.

Bake for 12-14 minutes or until cookies are just set and slightly golden brown at the edges.

Cool cookies on baking sheet for 5 minutes and transfer to cooking racks. Cool completely.

In a medium bowl, whisk together powdered sugar and lemon juice, a tablespoon at a time, until a very thick but pourable glaze forms.

Spoon each cookie with the lemon glaze. Garnish with extra lemon zest, if desired. Let cookies sit until icing has set, at least 30 minutes.

Cookies can be stored at room temperature in an airtight container for up to 3 days.