Line two baking sheets with parchment paper or silicone mats. Set aside. In a bowl, sift together the powdered sugar and almond flour. Set aside. In a separate large mixing bowl, beat the egg whites on medium speed until they become frothy.
Gradually add the granulated sugar to the egg whites while continuing to beat. Increase the speed to high and beat until stiff peaks form. Add the vanilla extract and gel food coloring (if using) to the beaten egg whites. Gently fold them in until well incorporated.
Gradually add the sifted powdered sugar and almond flour mixture to the egg white mixture. Gently fold the dry ingredients into the egg whites until the batter becomes smooth and shiny. This mixture is called macaronage. Transfer the macaron batter into a piping bag fitted with a round tip. Pipe small rounds onto the prepared baking sheets, spacing them about 1 inch apart.
Remove the scallops from the oven and let them cool for a few minutes. Carefully remove the scallops from the salt bed, using a spatula or tongs. Brush off any excess salt.
Tap the baking sheets on the counter a few times to remove any air bubbles. Let the macarons sit at room temperature for about 30 minutes to develop a skin. This will help them form their signature smooth shell. Meanwhile, preheat your oven to 325°F (160°C). Bake the macarons in the preheated oven for about 12-15 minutes, rotating the baking sheets halfway through baking. The macarons are done when they are firm to the touch and can be easily lifted off the parchment paper.
Remove the baking sheets from the oven and let the macarons cool completely on the sheets before removing them. Once the macarons have cooled, carefully remove them from the baking sheets. Match up macaron shells of similar size and shape. Fill a piping bag with your desired filling (e.g., buttercream, ganache, or jam). Pipe a small amount of filling onto the flat side of one macaron shell, then gently press another shell on top to create a sandwich.
Repeat the filling process with the remaining macaron shells. Place the filled macarons in an airtight container and refrigerate for at least 24 hours to allow the flavors to meld and the texture to soften. Bring the macarons to room temperature before serving. They can be stored in the refrigerator for up to 5 days.