Preheat the oven to 300° and have ready a large (18”) pastry bag, fitted with a plain tip, and your baking sheets lined with a silpat or parchment paper.
If you use almond flour, simply sift it with the powdered sugar and set aside. If a significant portion won’t go through your sifter, however, you’ll need to grind them up until they do.
If you are using whole almonds, process the almonds and powdered sugar for about a minute in a food processor. Take out the mixture and sift it, reserving whatever bits don’t pass through the sieve. Add these bits back to the food processor and run the machine for another minute. Sift again. You should have about 2 Tbsp of slightly chunkier almond bits. Just add those into the dry mix.
In the bowl of a stand mixer, combine the egg whites, sugar, vanilla bean, and salt and turn the mixer to medium (4 on a Kitchen Aid). Whip for 3 minutes.
Increase the speed to medium-high (7 on a Kitchen Aid) and whip another 3 minutes, then turn the speed to 10 for go another 3 minutes.
You should have a very stiff, dry meringue. If the meringue has not become stiff enough to clump inside the whisk, continue beating for another minute, or until it does so.
Now dump in the dry ingredients all at once and fold them in with a rubber spatula. Use both a folding motion (to incorporate the dry ingredients) and a rubbing/smearing motion, to deflate the meringue against the side of the bowl.
Essentially, the macaron batter needs enough thickness that it will mound up on itself, but enough fluidity that after 20 seconds, it will melt back down ("lava consistency").
Transfer about half the batter to a piping bag.
Pipe the batter into the baking sheet.
After piping your macarons, take hold of the sheet pan and hit it hard against your counter 3 times. Rotate the pan ninety degrees and rap two more times. This will dislodge any large air bubbles that might cause your macarons to crack
Bake for about 18 minutes, or until you can cleanly peel the parchment paper away from a macaron. If, when you try to pick up a macaron, the top comes off in your hand, it’s not done.
Once the macarons have baked, cool thoroughly on the pans, before peeling the cooled macarons from the pan. Use a metal spatula if necessary.
Fill a pastry bag fitted with the filling of your choice and pipe a quarter sized mound of filling into half of the shells, then sandwich them with their naked halves.