Instructions

Adjust an oven rack to the middle position and heat the oven to 375 degrees. Line 2 baking sheets with parchment paper.
Whisk the flours, baking powder, and salt together in a medium bowl; set aside. Using an electric mixer, cream the butter and 1 cup of the sugar together at medium speed until light and fluffy, 3 to 5 minutes, scraping down the sides of the bowl with a rubber spatula as needed. Add the egg, 2 teaspoons lemon zest, and vanilla and continue to beat at low speed until just combined, 30 to 60 seconds, scraping down the bowl as needed. Add the flour mixture and continue to beat at low speed until just combined, 30 to 60 seconds, scraping down the bowl as needed.
Process the remaining 1/3 cup sugar and 1 teaspoon grated lemon zest in a food processor until fragrant, about 10 seconds. Place the lemon sugar in a shallow bowl. Working with a level tablespoon of dough each time, roll the dough into 1-inch balls. (If the dough is too soft to roll, refrigerate it until firm.) Carefully roll the balls in the sugar and place them on the prepared baking sheets, spacing them about 2 1/2 inches apart (you will fit 12 cookies on each baking sheet).
Bake the cookies, one tray at a time, until the edges are light golden and the centers are just set, 9 to 11 minutes, rotating the tray halfway through baking (do not overbake). Cool the cookies on the baking sheets for 5 minutes, then serve warm, or transfer to a wrire rack and cool completely. Bake the second tray while the first tray cools.