How to Make It
Prepare the Macarons: Preheat oven to 300°F. Line 2 baking sheets with parchment paper. Beat egg whites in the bowl of a stand mixer fitted with the whisk attachment, until loose peaks form. Gradually add granulated sugar, and beat until sugar is dissolved and stiff peaks form.
Sift powdered sugar and almond flour through a fine mesh strainer. Fold flour mixture into whipped egg whites. Add food coloring until desired color is reached.
Transfer mixture to a ziplock plastic bag or piping bag with a rounded tip. Pipe 1-inch rounds onto baking sheets lined with parchment paper, and let stand in a dry place until rounds are dry to the touch, 1 to 2 hours.
Bake in preheated oven until ruffled edges (also called “feet”) have formed and macarons are set and not jiggly, 10 to 12 minutes.
Prepare the Filling: While macarons bake, beat together powdered sugar, butter, and milk with an electric mixer on medium until fully combined and smooth. Transfer filling to a piping bag.
Remove macarons from oven, and let cool on baking sheet set on a wire drying rack for 15 minutes, then transfer macarons directly to wire rack and continue cooling. Pipe or spread Filling onto 1 macaron, and top with a similar size macaron. Serve immediately or store in an air-tight container up to 2 days.