In a medium bowl, sift together the almond flour and powdered sugar, then set aside.
Make the macaron shells: In the bowl of a stand mixer fitted with the whisk attachment, whip the egg whites on medium speed until foamy. Continue to beat until your whisk begins leaving visible trails in the foamy egg whites.
Once you can see trails, gradually add the granulated sugar, increase the mixer speed, and whip on high until the meringue forms stiff peaks. Add gel food coloring in this stage, if using. (Be sure not to over-whip your egg whites otherwise you risk drying them out.)
Remove the bowl from the stand mixer, add the dry ingredients to the meringue and fold with a rubber spatula from the bottom of the bowl upward then press the flat side of your spatula through the middle against the side of the bowl. (The batter will look very thick at first, but it will get thinner as you fold.) Continue folding until the batter gets to a lava-like consistency.Pro Tip: The figure 8 test is a great way to check your batter’s consistency; pick up the batter with your spatula and let it flow down into the bowl while drawing the figure “8”. If it can do that without the batter breaking, immediately stop folding.
Transfer the batter into a large pastry bag with a medium-sized round tip.
Holding the piping bag at a 90 ̊ angle to the surface, pipe out the batter into 1.5-inch rounds about an inch apart on a baking sheet lined with parchment paper. (Feel free to print out a macaron template if you’re worried about size/uniformity). Add sprinkles on the top of each shell, if desired.
Holding the baking sheet with both hands, carefully bang the sheet firmly on the counter a few times to get rid of any air bubbles. (If you don’t release the air bubbles, they will expand during baking and crack your macaron shells.)
Repeat the piping and banging process until you have used up all the batter (usually about three sheet pans worth.)
Let the macarons rest and dry for 30 minutes or until a skin has developed before baking; on a humid day, it might take an hour or more. To see if it’s ready to be baked, lightly touch it. If the batter doesn’t stick to your finger, then it’s ready. (Don’t forget to remove the macaron templates, if using, before baking!)
While the macarons are resting, preheat the oven to 300˚F (150˚C) and position the oven rack in the center of the oven.
Bake the macarons, one tray at a time, for 18-20 minutes, rotating the pan once halfway through the baking process.
Remove from the oven and allow to cool on the sheet pan for 10 minutes before peeling off the parchment paper and cooling completely on a wire rack. (If the bottoms are a tiny bit sticky, keep them on the tray to cool off for an additional 10-15 minutes. If, however, the bottoms are already brown, they peel off cleanly, or they appear over-baked, then carefully take them off the tray immediately to cool down.) Repeat the baking process with the remaining sheet pans.Pro Tip: It’s always better to over- rather than under-bake your macarons as the maturation process can typically salvage ones that are over-baked.
Make the white chocolate swiss meringue buttercream: In the bowl of an electric stand mixer, whisk the egg whites and granulated sugar together briefly by hand, just until they are combined. Fill a saucepan with a few inches of water and bring to a simmer. Place the mixer bowl with the egg white mixture on top to create a double-boiler and heat the mixture until it reaches 160°F (70°C) and the sugar has dissolved, stirring constantly.
Carefully transfer the bowl to the mixer, and fit with the whisk attachment. Whip the egg whites on high until they are white and fluffy, and the mixture has cooled, 8-10 minutes. Add the butter one cube at a time. The mixture may look curdled - but just keep mixing! Once all the butter is incorporated, mix on high for 8-10 minutes. Add the vanilla and mix until incorporated.
Once the buttercream has finished mixing, and it's smooth and silky, add in the cooled white chocolate. Mix for 2-3 minutes to incorporate, then switch to the paddle attachment and mix on low for one minute to remove any excess air.
Make the cranberry filling: Place the ingredients into a small pot and bring to a boil over medium heat. Boil for 5 minutes until the cranberries burst and the mixture is syrupy. Blend until smooth, then allow to cool.
Transfer the white chocolate swiss meringue buttercream and the cranberry sauce into pastry bags and pair each macaron shell with another of a similar size. Pipe a ring of white chocolate buttercream on one shell, then fill it with the cranberry sauce before gently sandwiching together. Store the filled macarons in an airtight container in the fridge for up to 5 days and bring back to room temperature before enjoying.