Line two large baking sheets with parchment paper or silicon mats. Set aside.
Pour 110g of aged egg whites into the bowl of a stand mixer with a whisk and mix on medium speed until the surface of the egg whites is covered in small bubbles. Add in a pinch of cream of tartar and continue to mix until it reaches the soft peak stage where you can see the whisk leaving faint tracks in the egg whites.
Gradually add 110g of granulated sugar into the eggs and mix on a medium speed for 30 seconds. Increase the mixing speed to a medium-high speed. Keep mixing until stiff, glossy peaks form.
Sift 140g superfine almond flour and 126g powdered sugar into the meringue, then fold the ingredients together with a rubber spatula. If you want to color the shells like I did, add a small drop of yellow gel food coloring at this stage. Use a circular motion that sweeps around the edge of the bowl and then pull through the bottom of the bowl to make sure everything is getting mixed together.
Fold until a thick ribbon of batter runs off the spatula when it is lifted. You should be able to draw a couple of figure 8s with the batter running off your spatula when it is the right consistency. If the stream of batter breaks before you're able to do this, you may need to stir it a bit more.
Pour the batter into a large piping bag fitted with a medium-sized round piping tip and pipe 1 1/4-inch rounds on the prepared baking sheets, spacing them about 1 inch apart.
Pipe one pan at a time and bang the pan firmly on the counter a few times to release air bubbles, then pop any remaining air bubbles that come to the surface with a toothpick or scribe.
Let the macarons rest for 30 minutes, or until they develop a skin. The macarons should look matte once the skin has formed. While the macarons rest, preheat the oven to 315 F / 157 C (if you haven't already done so after baking the shortbread).
Bake one tray of macarons at a time on the middle rack of your oven for 17-21 minutes and rotate the pan halfway through to help them bake evenly. I like to place a foil tent over the macarons when I rotate them for the second half of the baking process to prevent them from browning.
Remove the pan from the oven and let the macarons cool on the pan (about 15 minutes), then gently remove them from the Silpat mat.