Step 1

Combine confectioners' sugar, almond flour, and 1/2 teaspoon salt in the bowl of a food processor. Pulse a few times until extra-fine. Sift through a fine-mesh sieve into a large bowl.

Step 2

Beat egg whites with the remaining 1/2 teaspoon salt and cream of tartar in the bowl of a stand mixer fitted with a whisk attachment until frothy. Gradually add superfine sugar and vanilla and continue to beat on medium speed until stiff peaks form. Do not overbeat; tips of peaks should hold their shape but curl over slightly.

Step 3

Gently fold 1/3 of the almond flour mixture into the egg whites, turning bowl occasionally, until nearly combined. Drop in food coloring. Add remaining almond flour mixture in 2 additions, gently folding until a batter comes together. Batter should fall off the spatula in thick ribbons without breaking.

Step 4

Transfer batter to a piping bag with a #804 pastry tip. Line 2 light-colored baking sheets with silicone macaron mats. Pipe batter into each circle on the mats. Tap baking sheets 4 times against the counter to release any air bubbles. Let macarons sit at room temperature until dry to the touch, 30 minutes to 1 hour.

Step 5

Preheat the oven to 300 degrees F (150 degrees C).

Step 6

Bake macarons in the preheated oven until bottoms rise slightly off the mats to form 'feet', 17 to 20 minutes. Switch baking sheets positions halfway through.

Step 7

Let macarons cool on the mats for 15 minutes. Transfer to a wire rack to cool completely, at least 15 minutes.

Step 8

Meanwhile, beat butter and confectioners' sugar together using an electric mixer until creamy. Add heavy cream and vanilla extract; beat until combined.

Step 9

Transfer buttercream to a piping bag with a #4 pastry tip. Pipe buttercream onto 1/2 of the macarons. Top with the other halves to create sandwiches.