Preheat the oven to 350 degrees and line two baking sheets with parchment paper.
Heat the chocolate in a saucepan over medium-low heat, stirring every 30 seconds, until melted. Set it aside.
Whisk the flour, cocoa powder, baking powder, and salt in a large bowl and set aside.
Add the butter and both sugars to a bowl and microwave for 90 seconds or until the butter is melted. Stir well.
Add the eggs, egg yolk, and vanilla to the butter mixture and whisk until combined.
Mix the melted chocolate into the wet ingredients with a rubber spatula until well combined.
Add the dry ingredients and crushed Oreos and mix with the spatula until just combined
Use a 2-tablespoon cookie scoop to add balls of cookie dough to the prepared baking sheets. Bake the cookies for 9-12 minutes.
Remove the cookies from the oven and let them cool on the baking sheets for 10 minutes. Then, transfer them to a wire cooling rack.
Oreo Buttercream
Whip the butter on medium speed in a large bowl for 1 minute.
Add the salt, vanilla, and half of the powdered sugar. Mix on medium for about 45 seconds until well blended. Add the other half of the powdered sugar and one tablespoon of milk at a time, mixing until the frosting is rich and creamy.
Add the Oreo crumbs and mix with a rubber spatula until completely combined.
Make the Sandwich Cookies
Turn half of the cookies upside down and frost the inside using a piping bag or spatula.
Top the frosted cookies with the other half of the cookies. Enjoy!
Video
Notes
Storage
Store at room temperature in an airtight container for up to 3 days.
Refrigerate for up to 5 days in an airtight container.
Freezing for Future Use
Freeze the cookies and frosting in separate airtight, freezer-safe containers for up to 3 months. Flash freeze the cookies, then wrap them in plastic wrap before storing.
Defrost the cookies and frosting overnight in the refrigerator. Then, whip the frosting again and make the cookies.
Tips
Microwave your butter and sugar mixture until it just melts. You want it to be liquid but not so hot that it scrambles the eggs when you add them.
Bringing your cold cookie dough ingredients to room temperature helps them mix more completely and more easily and avoids overmixing.
Mix your wet and dry cookie dough ingredients until just combined to avoid tough cookies.
A food processor is the easiest way to crush your Oreos for the cookie dough and the frosting.
The Oreos should be crushed into a very fine crumb for the frosting. This helps keep the frosting smooth and avoids clogging your piping tip.
For the smoothest frosting, sift your powdered sugar before using it.