How to Make the Cookie:

1. Prepare two baking sheets by lining them with parchment paper or silicone mats, then set aside.
2. In a medium-sized bowl, sift the powdered sugar and almond flour together, allowing it to fall through the sieve without pressing. Move this mixture to a food processor and pulse for 20 seconds. Pause to scrape down the sides, then pulse for an additional 10 seconds. Sift the mixture again into the same bowl and set aside.
3. In a large clean bowl or the bowl of a stand mixer equipped with a whisk attachment, combine the egg whites. Whip on medium speed for 30 seconds until foamy. Incorporate the cream of tartar and continue mixing on medium speed.
4. Gradually pour in the granulated sugar, one tablespoon at a time, while occasionally scraping the bowl to remove sugar buildup. Mix in the vanilla and food coloring if using. Whip until stiff peaks form, which should take about 6 minutes.
5. Gently fold in the almond flour mixture in three additions using a spatula. Once fully combined, press the mixture against the bowl’s side with the spatula to eliminate any large air bubbles; this process is known as macronage.
6. The batter is ready when it can fall off the spatula while maintaining a figure-eight shape. Transfer it to a piping bag fitted with a medium round tip (such as Wilton 1A).
7. Place a small amount of the mixture at each corner of your baking sheet to secure the parchment paper. Pipe one-inch dollops onto the sheet, leaving about 2 inches of space between each.
8. Gently tap the baking sheet on the counter to release any trapped air bubbles. (You can also use a toothpick to pop any remaining large bubbles). Let the piped batter rest for 40 minutes before baking, until it feels dry to the touch and looks less glossy.
9. As the resting time nears its end, preheat the oven to 270ºF. Bake for 18-20 minutes, rotating the pan 180º after 8 minutes. Allow the cookies to cool completely on the baking sheet before removing.

How to Make the Filling:

1. In a medium saucepan, combine the sugar and water. Heat over low while stirring until the sugar dissolves. Once dissolved, raise the heat to medium-high and bring to a boil.
2. While the sugar syrup is cooking, add the egg yolks to the bowl of your stand mixer fitted with a whisk attachment and beat until thick and foamy.
3. Continue to cook the sugar syrup until it reaches 240ºF. Remove from heat and, while the mixer is running, slowly drizzle the syrup into the egg yolks. Keep mixing until the bowl feels cool to the touch, which should take about 5 minutes.
4. With the mixer on medium-low speed, add the butter one tablespoon at a time, ensuring each addition is fully incorporated before adding the next. Mix in the vanilla and salt, continuing until the mixture is smooth and creamy, which should take about 5-6 minutes. If desired, add a few drops of food coloring and mix until combined.
5. Transfer the filling to a piping bag fitted with a small round tip (such as Ateco #10). Pipe onto the underside of half of the cookies, then top with an unfrosted cookie to create a sandwich. Store in the refrigerator for 1-3 days to allow the filling to soften the cookie shells.