In a dry mixing bowl, sift together the almond flour, the powdered sugar, and lemon zest.

Place the egg whites in a separate bowl and whisk until they appear frothy. Add the sugar to the egg whites and continue whisking until firm peaks form. (This is most conveniently done with an electric mixer/egg beater)

Add half of the dry flour/sugar mixture to the egg whites. Using a flat spatula, gently fold the egg whites back and forth until the dry ingredients have just incorporated into the mixture. Be careful not to overmix.

Scoop the liquid mixture into the dry bowl and continue folding back and forth, until the mixture has just incorporated itself.

Lay out parchment paper or silicone baking sheets on top of cookie sheets.

Scrape this mixture into a plastic piping bag (I used a large Ziplock freezer bag and cut a bottom corner off) and begin piping the mixture into small circles on the baking sheets. Be sure to space them about an inch apart because they will continue spreading after you have piped them out.

Let the batter sit on the sheets for 30-90 minutes. This allows the batter to rest and a smooth skin should form on the tops of the cookies.

The cookies are ready to be baked if you gently touch the tops of the cookies and no batter sticks to your finger. The surface should feel smooth and dry. I would check every 30 minutes to make sure.

Heat the oven to 280F (~140C). When the oven is done preheating, place the sheets in the oven and bake 17 minutes.

Remove the cookies and let cool (5-10 minutes). Carefully remove from the cookie sheet and place upside down for filling. Top the filling with another cookie half.

Fill with Lemon Curd.

Eat! The cookies keep well at room temperature for about 3 days.