Place egg whites into a metal mixing bowl and refrigerate overnight. The next day, bring egg whites to room temperature.
Preheat oven to 280 degrees F (138 degrees C). Line baking sheets with parchment paper.
Whisk confectioners' sugar and almond flour in a bowl. Beat the egg whites with salt in metal bowl with an electric mixer on medium speed until foamy, about 1 minute; increase speed to high and gradually beat in superfine sugar, about 1 tablespoon at a time, until the egg whites are glossy and hold stiff peaks, 3 to 5 more minutes.
Gently fold almond flour mixture into whipped egg whites until thoroughly incorporated; spoon meringue into a pastry big fitted with a 3/8-inch tip.
Pipe 1-inch disks of meringue onto the prepared baking sheets, leaving 2 inches of space between cookies. The batter will spread. Lift the baking sheets a few inches above the work surface and hit them lightly on the work surface several times to remove any air bubbles from the cookies. Let the cookies stand at room temperature until the shiny surfaces become dull and a thin skin forms, about 15 minutes.
Place the baking sheets in the preheated oven and bake with the oven door open slightly until the macarons' surfaces are completely dry, about 15 minutes. Let cookies cool completely on a baking sheet before peeling parchment paper from the cookies.
Spread half the cookies with any desired filling, top with remaining cookies to make sandwiches, and refrigerate at least 2 hours to overnight to let the cookies soften.
