Place the almond flour and 1-1/2 cups plus 3 Tbsp. confectioners’ sugar in a food processor; pulse until thoroughly mixed to ensure almond flour is very fine. Pass almond flour mixture through a fine-mesh sieve; discard any large pieces that remain.
Place egg whites and salt in a very clean bowl of a stand mixer fitted with whisk attachment; whisk on medium-low speed until frothy. Slowly add superfine sugar; whisk until dissolved, 1-2 minutes. Slowly add remaining confectioners’ sugar; increase speed to high and whip until meringue is glossy and stiff peaks form, 2-3 minutes.
Gently fold a third of the almond flour mixture into meringue; then gently fold in remaining almond flour in 2 additions. Using side of spatula, smooth batter up sides of bowl several times to remove air bubbles and ensure there are no lumps; do not overmix. Run spatula down the center of the bowl; the line in the batter should remain visible for a moment before mixture runs back into itself.
Position rack in upper third of oven; preheat oven to 300°. Transfer batter into a pastry bag fitted with a #7 or #10 round tip. Pipe 1-3/8-in. rounds onto parchment about 1 in. apart. Tap tray against counter 2-3 times to remove excess air bubbles. Let macarons rest until no longer wet or sticky to the touch, 30-60 minutes. Bake, 1 tray at a time, until cookies rise about 1/8 in. to form "feet," 14-16 minutes, rotating tray halfway through baking. Remove tray and let macarons cool completely; repeat with remaining trays. Once macarons have cooled completely, remove from parchment.
To make filling, cream butter in a stand mixer fitted with whisk attachment; slowly add confectioners' sugar until incorporated. Add heavy cream, vanilla and salt; mix until smooth. Pour frosting into a pastry bag fitted with a small round tip; pipe buttercream onto half the macarons. Top with remaining macaron shells. Refrigerate, covered, until ready to serve.
Peppermint Variation: For macaron shells: Add green gel food coloring (do not use liquid food coloring) to whipped meringue until desired color is reached. To decorate: If desired, with crushed candy canes, top macaron shells just after they have been piped and before skin has formed. For filling: Add 1/4 to 1/2 teaspoon peppermint extract to mixed frosting. Assemble as directed.
Cranberry Variation: For macaron shells: Add red gel food coloring (do not use liquid food coloring) to whipped meringue until desired color is reached. To decorate macaron shells: Place white candy melts or white chocolate in a microwave-safe bowl and microwave at 30-second intervals, stirring frequently, until melted and smooth. Place chocolate in a piping bag fitted with a fine round decorating tip; drizzle over macaron shells. Immediately sprinkle with red, green and white assorted sprinkles. For filling: If desired, add 2-3 drops cranberry flavoring to frosting. Pipe a circle of frosting onto bottoms of half the macaron shells. Place 1/4 teaspoon canned cranberry sauce in center of each frosting circle. Top with remaining macaron shells.
Hot Chocolate Variation: For macaron shells: Add 2 tablespoons dark cocoa powder to almond flour and confectioners' sugar before sifting. To decorate: Using stiff royal frosting, pipe snowflake designs onto half of the cooled macaron shells. For filling: If desired, using marshmallow fluff instead of frosting, pipe a circle onto bottoms of half the macaron shells. Place 1/2 cup chocolate chips and 2 tablespoons heavy cream in a microwave-safe bowl; microwave at 30-second intervals, stirring frequently, until melted and smooth. Place 1/4 teaspoon chocolate mixture in the center of each marshmallow circle; top with remaining macaron shells.