While cookies made from almond flour and egg whites have been around since at least the 16th century, the concept of filling them and sandwiching them together to create what we know as a macaron is a 20th century invention, popularized by the Parisian pastry shop Ladurée. Typically the shells are colored to reflect the flavor of the filling, which can be anything from buttercream to ganache to jam. You can eat the filled cookies right away, but they’re best after a day or two, as the shells will soften slightly and absorb the flavor of the filling, yielding a tender, pillowy cookie.
Heat oven to 350°. Line 2 baking sheets with parchment paper and set aside. Combine confectioners’ sugar and almond flour in the bowl of a food processor and pulse until fine. In a separate bowl, beat egg whites until soft peaks form; gradually add in sugar and continue beating until medium-stiff peaks form. Add food coloring, if desired. Carefully fold dry ingredients into meringue. Place batter into a piping bag fitted with a ½ » tip. Pipe batter onto prepared baking sheets into 1″ circles about 1″ apart. Tap the baking sheet a few times firmly on the countertop, then bake for 15–18 minutes. Let cool completely. Fill with buttercream and sandwich cookies together. Store filled cookies uncovered in the fridge for 1–2 days before consuming. Filled cookies can be frozen for up to 1 month.