Gather the ingredients. I had already zested the lemon before taking this picture. 🤪

Whisk together the flour, cornstarch, and sea salt in a bowl. Add the lemon zest.

Finely mince the fresh rosemary and add it to the dry ingredients. Whisk well to combine.

Cream the butter and powdered sugar. I used a stand mixer fitted with a paddle attachment and creamed for about 5 minutes at medium speed, but you could also use a hand mixer, or cream the butter and sugar by hand. Just be sure to scrape the bowl often. You want the end result to be a smooth paste.

Add the dry ingredients, and mix gently until they are fully incorporated. Don’t over mix. The end result should be a pliable dough that you can smooth out with your fingertips.

Divide the dough in half, and roll each portion into a log approximately 8 inches long. Wrap each log tightly in plastic wrap and refrigerate overnight.

I rolled one of the logs in granulated white sugar. This is optional.

Trim the rounded ends off of the logs. Use a ruler to mark 1/4 inch increments. Then, cut each log into 1/4 inch (6 mm) slices. Place the cookies onto ungreased baking sheets. I placed the baking sheets into the fridge to chill until the oven was preheated to the proper temperature.

Preheat the oven to 350°F (180°C)

Bake the cookies for 11 to 15 minutes. Shortbread cookies have a narrow window of doneness, so start checking the cookies after 11 minutes. They will be done when the edges are very lightly brown, and the bottoms are a nice golden color.

Remove the cookies from the oven, let them rest on the cookie sheets for about a minute, then transfer them to wire racks to finish cooling.

Store the cookies in an airtight container for up to 5 days.