I've read multiple recipes that ask for aged egg whites, some for up to 3 days. I've tried the recipe with fresh egg whites brought up to room temperature and aged ones and didn't see a big difference. I think the most important part is that they are not cold and straight out of the refrigerator!

For the berry flavor in this recipe, I'm using freeze-dried fruit, which I found at Trader Jo's. Simply place the berries into a food processor, pulse for a few seconds and sift through fine mesh strainer

Now, I'm going to place my egg whites into my mixer bowl and add my sugar. Turn your mixer on high and whisk until stiff peaks form. (turn the bowl over, egg whites should not move)!

Sift into a large bowl the flour, confectioner's sugar and fruit powder. Gradually fold in the flour into the egg whites. Mixture needs to have a specific consistency. Not too thick or too thin, should fall off of your spatula in a ribbon.

I've lined 2 baking sheets with parchment paper and preheated my oven to 300F. The first time I made these cookies, I set my oven to 325F and they started to burn. Watch your cookies when they're in the oven and open the door if you think you see any color on them!

Transfer batter into a pastry bag tipped with a round tip. If you have the time, you can draw circles on the back of the parchment paper to use as guides. Otherwise, just freehand it!

Pipe onto your parchment paper. Tap pan multiple times to release any air bubbles. Then, let pan stand for 1 HOUR (very important, do not skip this step!) to form a thin film over the top.

I bake my cookies for 15 minutes.

Once you take them out, let them stand for a few minutes before removing them. Let them cool completely before filling.

For filling: Whisk the butter in medium-sized mixing bowl for a few minutes until fluffy. Add lemon zest, lemon juice, and confectioner's sugar until buttercream is the right consistency. Pipe onto one cookie, top with another.