Step 1: Prepare the Macaron Shells

1. Sift the Dry Ingredients: In a medium bowl, sift together the powdered sugar and almond flour to ensure no lumps. Add the finely ground Oreo crumbs and gently mix until combined. Set aside.
2. Whip the Egg Whites: In a clean, dry bowl, whisk the egg whites on medium speed until foamy. Add a pinch of cream of tartar and gradually start adding the granulated sugar, a little at a time. Continue beating until you achieve stiff, glossy peaks. This usually takes about 5-7 minutes.
3. Fold in the Dry Ingredients: Using a spatula, gently fold the almond flour mixture into the beaten egg whites. The batter should fall off the spatula in a ribbon and disappear back into the mixture after a few seconds.
4. Pipe the Macarons: Transfer the batter into a piping bag fitted with a round tip. Pipe 1-inch circles onto a baking sheet lined with parchment paper or a silicone baking mat.
5. Rest the Macarons: Tap the baking sheet firmly against the counter a few times to release any air bubbles. Let the macarons sit at room temperature for 30-60 minutes, or until a skin forms on the surface.
6. Bake the Macarons: Preheat your oven to 300°F (150°C). Bake the macarons, one tray at a time, for 15-18 minutes. They should rise and develop their signature feet. Let them cool completely before removing from the baking sheet.

Step 2: Make the Oreo Buttercream

1. Beat the Butter: In a large bowl, beat the room temperature butter until light and fluffy.
2. Add the Sugar: Gradually add in the powdered sugar, beating well after each addition. Add the vanilla extract and crushed Oreo crumbs. If the mixture is too thick, add heavy cream, one tablespoon at a time.

Step 3: Assemble the Macarons

1. Pair the Shells: Match up the macaron shells by size.
2. Fill the Macarons: Pipe the Oreo buttercream onto the flat side of half of the macaron shells. Gently press the other half of the shells on top.
3. Chill and Serve: For best results, refrigerate the macarons for 24 hours before serving.