In a food processor, pulse confectioners' sugar and almond flour until combined. Sift mixture 2 times to remove any larger clumps. Set aside.
In the bowl of a stand mixer fitted with the whisk attachment, whisk egg whites on medium speed until foamy. Add cream of tartar, and whisk until soft peaks form.
Reduce speed to low, then add superfine sugar. Increase speed to high, and whisk until stiff peaks form, about 8 minutes.
Turn off the mixer and sift flour mixture over whites. Use a rubber spatula to fold until mixture is smooth and shiny. (About 40 folds, it will be a lava-like consistency).
Transfer batter to a pastry bag fitted with a 1/2-inch plain round tip, and pipe 3/4-inch rounds 1-inch apart prepared baking sheets. Drag the pastry tip to the side of rounds rather than forming peaks.
Tap bottom of each sheet on work surface to release trapped air bubbles. Let stand at room temperature for 20-30 minutes. Reduce oven temperature to 325° F. Bake 1 sheet at a time until macarons are crisp and firm, about 10 minutes. Avoid opening the oven door to check on them. After each batch, increase oven temperature to 375° F, heat for 5 minutes, then reduce to 325° F and bake next tray.
Let macarons cool on sheets for 2 to 3 minutes, then transfer to a wire rack. Prepare filling.
Prepare a small pot of simmering water.
In a bowl of a stand mixer, whisk together egg whites, sugar and salt. Set bowl over the pot with simmering water and whisk constantly until the sugar dissolves. You know it’s ready with it looks like marshmallow fluff.
Transfer the bowl back to the stand mixer and use the whisk attachment to beat the egg whites into stiff peaks, about 10 minutes. The bowl should be cool to the touch before you move on.
Switch over to the paddle attachment and set the mixer to medium speed. Add the butter one tablespoon at a time, letting it incorporate before adding another piece. Turn the mixer up to medium-high speed and beat until the buttercream comes together. This will take a while. If you are having trouble reaching a good consistency, place bowl in the fridge for a bit and then try again.
Once the buttercream is smooth and together, add peppermint extract or eggnog filling ingredients and mix on low speed until combined.
Place buttercream into a piping bag and pipe a small circle onto one macaron shell. Top it with a similar sized shell and press together carefully.