With the whisk attachment, whip the egg whites until foamy, 3-5 minutes. Usually on speed 4 or 6.
Add the cream of tartar and salt and continue whipping until soft peaks form.
Add the food coloring if you'd like (and yes: it does matter the type of food coloring as non-gel food coloring is a lot wetter and will change the consistency)
Add the baker's sugar a tablespoon at a time until dissolved.
While the mixer is going, sift together your almond flour and powdered sugar. A couple of times. Discard any chunks.
Line 2 baking sheets with parchment paper.
When all the sugar has been added to the meringue and it forms hard peaks, remove bowl from the stand mixer and sift the flour/sugar mixture into the meringue and, using a silicon spatula, incorporate it the flour/sugar into the meringue until (a) all sifted in and (b) dropping a bit of the mixture back onto itself it re-incorporates itself within 10-20 seconds. If the drop doesn't blend back into the mixture it needs more mixing.
Fill a piping bag with a Wilton #2A tip with the mixture and tie off.
Holding the pastry bag vertical to the baking sheets and parchment paper pipe out 1-1.5" rounds. Try to be consistent -- I dare you. I shoot for 4 per row, 5 rows per baking sheet -- they don't spread much -- so you should be able to get 40-50 rounds total. (Note: I usually do one oblong one at the end of one sheet as a tester to touch for later.)
Bang the sheets on the countertop to get any air bubbles out of the shells. Good luck with this, too. Pop any large air bubbles with a toothpick and then bang on the counter again.
Let the shells sit on the counter, uncovered, for 30-60 minutes and maybe a couple of hours (depends on humidity). What are you looking for? When you touch the tops (or the tester as I alluded to above) the batter doesn't stick to your finger.
Preheat oven to 300°F (150°C).
Bake one sheet at a time for 16-19 minutes, turning halfway through. To test for doneness touch and try to move -- it if wobbles, go another minute or two. Honestly, over-baking is better than under-baking.)
Let cool completely. COMPLETELY. Removing -- or trying to remove -- too early may result in breakage and stickage. Then remove from the parchment paper to a cooling rack and cool even more.
Since I suck at making them a consistent size, once completely cooled I match up similarly-sized shells to be sides.
Make a buttercream filling (see recipe) and pipe onto one half. Mash together. Let set in the refrigerator for 12-24 hours to "ripen." Remove from refrigerator 30-40 minutes before serving.