Line 3 baking sheets with parchment paper. If you wish you can use a pencil to trace out the circles
Sift the almond flour, oreo powder and powdered sugar twice in a medium bowl.
In the bowl of a stand mixer with the whisk, whip the egg whites on medium until it starts to foam. Raise the mixer speed to high, and slowly stream in the granulated sugar. Continue whipping until you get medium peaks, about 5-7 minutes.
Gently fold in half of the almond mixture and your vanilla. Once combined, add the remaining almond mixture and continue folding. When folding, you want to be bringing the batter around the sides of the bowl in order to deflate the batter. Continue doing this until your batter looks like lava
Transfer the batter to a pastry bag and pipe out the cookies onto your sheets. Bang (not too hard) each pan twice on your counter, this will get rid of any air bubbles.
Let the macarons stand at room temperature for 20-30 minutes, until they form a skin. If you touch them and they are no longer sticky, then they are ready to bake! Depending on the weather, it can take much longer for them to form a skin.
Preheat the oven to 300°F/150°C. Bake them one sheet at a time in the middle of the oven, for 15-17 minutes
Make filling
In a small saucepan, melt the butter over a medium-low heat. We don't want to brown the butter! Let the butter simmer until it stops sputtering. Once the butter is silent, pour into the bowl of a stand mixer that is fitted with a paddle attachment.
Add the vanilla extract, salt and icing sugar. Mix on low to get all the ingredients moistened. Increase the speed to the medium and beat until soft, about 5 minutes. Fill a piping bag with the filling and use immediately. Pipe 25 cookies with the filling and sandwich them.
Notes
The filling recipe is adapted from BraveTart. US Cups: I do not include cup measurements for this recipe, as you do need to be precise when making macarons. Aged Egg Whites: If you want to age your egg whites (not a requirement but it can help!), separate your egg whites 24 hours before and place in a bowl in the fridge. Take them out 2 hours before making your macarons so they can come to room temperature.Storage: Keep your macarons in a container in the fridge for up to one week.