How to Make It
Preheat the oven to 325º.
Pulse the confectioners' sugar and almond flour in a food processor until combined. Sift the mixture 2 times, discarding any remaining almond chunks.
In a large mixing bowl or standing mixer, whisk the egg whites on medium until soft peaks form. Slowly add the granulated sugar (and food coloring if using) and increase speed until stiff peaks form. Fold the sifted flour and powdered sugar mixture into the whites, until mixture is smooth and shiny.
Transfer the batter to a pastry bag fitted with a 1/2-inch plain round tip, and pipe 3/4-inch rounds 1 inch apart on a parchment-lined baking sheets. Tap the bottom of each sheet on the work surface to release any trapped air.
Let this stand at room temperature for 10 minutes, then bake in oven, one sheet at a time, rotating halfway through, until macarons are cooked through, about 10 minutes. Let the macarons cool on the sheet for two minutes, then transfer to a wire rack to cool completely.
In a food processor or stand mixer, beat together the butter, confectioners' sugar, orange zest, orange juice and sea salt until smooth and creamy. Place in a pastry bag fitted with a 1/2-inch plain round tip.
To complete the macarons, pipe one teaspoon of buttercream filling onto one shell and top with another. Serve immediately, or stack between layers of parchment, wrap in plastic and freeze for up to 3 months.