Ingredients (around 12 sandwich cookies):
Line two large baking sheets with parchment paper.
Process powdered sugar and almond flour in a food processor until finely ground. Sift the mixture through a sieve. If there are more than 2 tablespoons of large chunks left in the sieve, grind them and sift again. Set the mixture aside.
In the bowl of a stand mixer fitter with a whisk attachment, beat egg whites at medium speed until frothy. Reduce the speed to low and gradually add the granulated sugar, 1 tablespoon at a time. Increase the speed to high and beat until very stiff peaks form, about 5 minutes.
Sift the almond flour mixture over the egg whites. Using a rubber spatula, gently fold the almond flour mixture into the egg whites until the mixture loosens and falls in a ribbon from the spatula.
Fit a pastry bag with a 1/2-inch round tip and fill the bag with the batter. Pipe Os to create donut shapes onto the parchment papers, leaving at least one inch apart.
Tap the bottom of each sheet on the work surface to release trapped air bubbles. Let the cookies stand at room temperature for at least 30 minutes to develop their crusts.
Preheat oven to 320F. Bake the macarons for 10 to 12 minutes, until set but not browned.
Transfer the baking sheets to wire racks and let the macarons to cool completely on the pans. Once cooled, gently lift half of the cookies from the parchment paper and turn them upside down.
Pipe a teaspoon of filling onto each of the upside-down cookies.
To prepare the glaze, whisk sugar, milk and vanilla slowly until slightly thickened. Add more milk or sugar to get the right consistency. Add coloring paste, one drop at a time, and mix until color is evenly distributed.
Dip the top of each of the remaining cookies in the glaze and set on a rack placed over a baking sheet. Decorate with sprinkles immediately. Let dry completely, about 10 minutes. Place it on top of the filled cookie.