Preheat oven to 300°
Line 3 baking sheets with silicone mats. Measure the confectioners’ sugar and almond flour by spooning thei into measuring cups and leveling with a knife. Transfer to a bowl; whisk to combine.
Sift the sugar-almond flour mixture, a little at a time, through a fine-mesh sieve into a large bowl, pressing with a rubber spatula to pass through as much as possible.
Beat egg whites, cream of tartar and salt with a mixer on medium speed until frothy.
Increase the speed to medium high; gradually add the superfine sugar and beat until stiff and shiny, about 5 more min.
Transfer the beaten egg whites to the bowl with the almond flour mixture.
Draw a rubber spatula halfway through the mixture and fold until incorporated, giving the bowl a quarter turn with each fold.
Add the food coloring extract continuously folding, turning and scraping down until the batter is smooth and falls of the spatula in a thin ribbon.
Transfer the batter to a pastry bag fitted with a ¼ inch round tip.
Holding the bag vertically and close to the baking sheet, pipe 1 ¼ inch circles (24 per sheet).
Firmly tap the baking sheets twice against the counter to release any air bubbles.
Let the cookies sit at room temperature until the tops are no longer sticky to the touch, which should be 15 minutes to 1hr, depending on the humidity.
Slip another baking sheet under the first batch (a double baking sheet protects the cookies from the heat).
Bake the first batch until the cookies are shiny and rise 1/8 inch.
Peel the cookies of the mats and sandwich with thin layers of filing.