Place the oreo shells in the bowl of a food processor and pulse on high speed until they are a fine crumb. Set aside until ready to use.
Line two large sheet pans with parchment paper.
Place a small saucepan of water over medium-high heat and bring it to a gentle simmer. Add the egg whites and granulated sugar to a small heatproof bowl and set the bowl of the simmering water.
Whisk the egg whites and sugar together until the sugar has fully dissolved-- about 3-4 minutes.
Transfer the mixture to the bowl of a stand mixer fitted with the whisk attachment. Beat the egg whites on medium high speed for 3-4 minutes, or until they form stiff peaks.
Use a fine mesh sieve to sift the powdered sugar and almond flour into the egg whites. Gently fold the dry ingredients into the egg whites, making sure not to deflate the egg whites yet.
Once all of the dry ingredients have been incorporated (make sure to scrape all the way down to the bottom of the bowl), smush the batter against the sides of the bowl and fold it back together. Repeat the smush-and-fold process 4-5 times before testing the macarons for the lava stage.
The meringue is ready to pipe when it flows smoothly off the spatula. If it falls off in clumps, do a few more smush-and-folds before retesting. When the meringue flows smoothly, transfer it to a piping bag fitted with a round tip (I like a Wilton #10).
Pipe small (1.5”) macarons onto the sheet pans making sure to pipe them about 2” apart. Once you’ve finished piping the first tray, hold it about 6” off the counter and drop it straight down to release any air bubbles. Drop the tray 4-5 more times, or until it looks like any large air bubbles have popped. Repeat with the second tray.
Sprinkle about ¼ cup of oreo powder over the macarons and set them aside to rest for 25-30 minutes.
The macarons are ready to bake when they have developed a skin-- you should be able to touch them gently without the meringue sticking to your finger.
While the macarons rest, preheat your oven to 325F.
Bake the macarons, one tray at a time, for 14 minutes, turning the tray halfway through baking.
Allow the macarons to cool fully on the tray. Trying to remove the macarons while they are warm may result in sticking.