Line two to three baking sheets with non-stick silicone baking mats or parchment paper and set aside. Fit a large piping bag with a round tip, ¼ to ½-inch wide in diameter, and set aside.
Place the almond flour and confectioners’ sugar in a food processor. Grind until fine, 30 to 60 seconds. Using a fine mesh sieve, sift the almond flour mixture into a large mixing bowl. Press any clumps through the sieve with a rubber spatula.
Place the granulated sugar and water in a saucepan and gently swirl together. Heat the sugar mixture on the stove over high heat. Without stirring, bring the sugar mixture to a boil. Keep boiling until the sugar syrup reaches 238°F on a candy thermometer, or soft ball stage.
Meanwhile, place half of the egg whites (about 52-53 grams) in the bowl of a stand mixer fitted with a whisk attachment. Once the sugar syrup reaches 230°F, begin whipping the egg whites on medium-high speed until they turn white and hold soft peaks - like wispy clouds.
Once the sugar syrup reaches 238°F and the egg whites are whipped to soft peaks, very carefully pour the sugar syrup into the egg whites with the mixer running on high speed. Pour the hot syrup in a steady stream - not too slow that it dribbles and spills, and not too quickly that you lose control or shock the delicate egg whites.
After the sugar syrup has been added to the egg whites, continue to whisk on high speed until stiff peaks form and the meringue returns back to room temperature, 6 to 8 minutes. Add gel food coloring to the meringue during the last minute of mixing.
Meanwhile, add the remaining egg whites (about 52-53 grams) to the sifted almond flour mixture. Stir the ingredients together with a rubber spatula until the mixture turns into a paste. Press and smear the paste against the side of the bowl to remove lumps.
Add about ⅓ of the meringue to the almond mixture and mix until they just start to combine. You don’t need to be as delicate here. Stop before the two mixtures are fully combined.
Continue to fold all of the meringue into the batter (see Macranoge for more info). Do not over-mix. The batter is done mixing when you can lift up a small portion with the spatula and slowly wave it back and forth, like a ribbon, without it breaking. The “ribbon” should melt back into the batter in about 30 seconds.
Fill the prepared piping bag with the macaron batter. Holding the bag straight down, pipe small, uniform mounds on the lined baking sheets. The macarons should be 1 to 1 ½-inches in diameter, piped about an inch apart. Sprinkle with rainbow sprinkles, if desired.
After one full baking sheet is piped, it is time to tap the bottom of the pan. This releases any trapped air bubbles and helps any peaks left from piping settle flat. (See Pipe and Tap).
Preheat the oven to 325°F and set the piped macarons aside to rest. Rest the macarons until a skin forms on their tops, 20 to 45 minutes. They are ready to bake once the oven is preheated and the tops are dry and not sticky when gently touched with a finger.
Bake one baking sheet at a time (or two small sheets, if they fit on the same rack) for 12 to 14 minutes. Cool macarons completely on their baking sheets set over a wire cooling rack.