In a standing mixer using a whisk attachment (or mixing bowl with a hand mixer), whip the egg whites and salt on high speed for about 2 minutes, until soft peaks form. Add in granulated sugar and chocolate extract and continue to beat for 1 minute until very stiff peaks form. The egg whites should be foamy and firmly hold their shape.
In a small blender, pulse your oreo cookies until they form a fine powder. Using a fine mesh strainer, sift the almond flour, powdered sugar, and powdered oreos three into a medium bowl. Sift the mixture again two more times. You should be left with small almond granules each time you sift. Toss these out after each sifting.
Carefully pour the powdered sugar mixture on top of the whipped egg whites. Fold a spatula into the center of the mixture, scooping down to the bottom of the bowl, and then scooping upwards along the side of the bowl. You should be making a rough circle with your spatula: center, down, out, center, down, out. Slowly but surely, your two mixtures should come together. Continue to fold the mixture until it resembles molten lava- when you pick the mixture up with your spatula, large ribbons should fall down into the bowl, unbroken by the air, and the entire mixture should slowly seep downwards like molten lava.*
Fit a large pastry bag with a 1/2 inch tip and scoop your mixture into the bag. Pipe 1 1/4 inch circles onto 2-3 parchment-lined cookie sheets, leaving at least 1 1/4 inches between each macaron. The best way to make even circles is to pipe from directly above the macaron so your pastry tip is perpendicular to the cookie sheet. Squeeze the pastry bag from the top with one hand while using your other hand to steady the tip. Once you’ve piped your circle, swirl the pastry tip in a small spiral and lift upwards at the same time to finish your macaron.* Allow macarons to air out for at least 1 hour, and up to 2 hours.
Preheat oven to 300°F. Bake macarons for 17 minutes. Allow to cool completely on their tray before frosting.
Now make your buttercream. Beat the butter with the oreo frosting and vanilla extract until light and fluffy, about 2 minutes. In a small blender, pulse your oreo cookies until they form a fine powder. Add in the crushed oreos, powdered sugar, and whole milk, beating again until light and fluffy, 1 minute more. Transfer to a small pastry bag fitted with a 1/2 inch tip. Once your macarons have cooled, frost and serve!
*There are a ton of video tutorials out there that show good technique for all stages of macaron making- check them out!