Line baking sheet with a silocone mat or parchment.*
Place the almond flour, powdered sugar, and Oreo cookie crumbs in a food processor. Process until just beginning to clump. Sift and discard any bits that won't pass through the sifter. Set aside.
Whip the egg whites until they're the consistency of a loose foam, add in cream of tartar. Continue whipping while slowly adding caster sugar. Whip to a stiff meringue. Stop as soon as the meringue is stiff, no not overwhip.
Fold the dry mixture into the meringue until it reaches a thick lava-like consistency.**
Pour the batter into a piping bag fitted with a 1A piping tip.
Pipe the macarons onto your baking sheet. Sprinkle with Oreo cookie crumbs if desired.
When all your macarons are piped, whack the baking sheet on the counter at least 3 times to get rid of any air bubbles.***
Let the macarons dry at room temperature until dry to the touch. About 1 hour. DO NOT skip this step.
Once dry, bake macarons in a preheated 300°F oven until they stick just slightly to the pans. This can be anywhere from 10-20mins depending on your oven and how crowded the macarons are on the pan.
Let cookies cool completely before removing from pans.