Place the almond flour and 1-1/2 cups plus 3 Tbsp. (175 grams) confectioners’ sugar into the bowl of a food processor; pulse until thoroughly mixed and 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 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 1/3 of the almond flour mixture into the meringue; gently fold in remaining almond flour in two 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 over-mix. When the spatula is run down the centre of the bowl, the line created in the batter should remain visible for a moment, before mixture runs back into itself.
Position rack in oven to upper third; preheat oven to 300°. Fit a #7 or #10 round pastry tip inside a pastry bag; gently pour batter into pastry bag. Pipe 13/8-in. rounds onto parchment paper about 1-in. apart. Tap tray against counter two to three times to remove excess air bubbles. Let macarons rest until no longer wet or sticky to the touch, 30-60 minutes. Bake, one tray at a time, until cookies rise about 1/8-in. to form ‘feet’, 14-16 minutes, rotating tray half way through cooking. Remove tray and let macarons cool completely; repeat with remaining trays. Once macarons have cooled completely, remove from parchment paper.
To make filling, cream butter in a stand mixer fitted with whisk attachment; slowly add powdered 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 of the macarons. Top with remaining macaron shells. Refrigerate, covered until ready to serve.
Peppermint Variation: For macaron shells: add green gel food colouring (do not use liquid food colouring) to whipped meringue until desired colour is reached. To decorate: if desired, top macaron shells with crushed candy canes just after they have been pipped and before skin has formed. For filling: add 1/4-to-1/2 teaspoon peppermint extract to mixed frosting. Assemble as usual.
Cranberry Variation: For macaron shells: add red gel food colouring (do not use liquid food colouring) to whipped meringue until desired colour 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 in a disposable pipping 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 of the macaron shells. Place 1/4 teaspoon canned cranberry sauce in centre of 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 of the macaron shells. Place 1/2 cup chocolate chips and and 2 tablespoons heavy in a microwave safe bowl; microwave at 30 second intervals, stirring frequently, until melted and smooth. Place 1/4 teaspoon chocolate mixture in the centre of marshmallow circle; top with remaining macaron shells.