1-Preparing the Macaron Shells: First, line two baking sheets with parchment paper or silicone mats to prevent sticking. Whisk the 110 grams of aged egg whites at medium speed until small bubbles form, then add the 1/4 teaspoon of cream of tartar and mix until soft peaks appear. Gradually add the 110 grams of granulated sugar, mixing on medium speed for 30 seconds before increasing to medium-high speed until you get stiff, glossy peaks.

2-Preparing the Macaron Shells: Next, sift the 140 grams of superfine almond flour and 125 grams of powdered sugar into the meringue. Fold everything together gently with a circular motion until the batter flows like a thick ribbon. Pipe 1 3/4-inch rounds onto the baking sheets, spacing them 1 inch apart, and bang the pans on the counter to release air bubbles. Use a toothpick to pop any remaining bubbles and sprinkle the 40 grams of nonpareil sprinkles on the wet shells.

3-Preparing the Macaron Shells: Let the shells rest for 30 minutes or until a skin forms and they look matte. Preheat your oven to 315 degrees Fahrenheit and bake one tray at a time on the middle rack for 18-22 minutes. Rotate the pan halfway through and cover with foil to avoid browning. Cool the macarons on the pan for 15 minutes before removing them.

4-Making the Buttercream Frosting: To prepare the buttercream, beat the 113 grams of unsalted butter at room temperature on medium speed until it’s light and smooth. Add the 4 grams of vanilla extract and 2 grams of fine salt on low speed, then slowly mix in the 250 grams of powdered sugar and 15 grams of heavy whipping cream until combined.

5-Making the Buttercream Frosting: Divide the frosting into six portions and color each one with gel food coloring in red, orange, yellow, green, blue, and purple. Spread or pipe long rows of the colored frosting on plastic wrap, roll it into a sausage shape, twist the ends, cut one end, and place it into a piping bag with a large round tip.

6-Assembling the Macarons: Pair up the cooled macaron shells and pipe a dollop of the rainbow buttercream onto one shell, then press the matching shell on top to form a sandwich. Store them in an airtight container, chill in the fridge overnight, and let them sit at room temperature before serving for the best flavor.