Design Eat Repeat

0 Preheat oven to 350°F.
2 Warm the unsalted butter [adjustable](½ cup, i.e. 1 stick)[/adjustable] in a microwave-safe bowl or saucepan until it’s just melted.
3 Combine the melted butter with the light brown sugar [adjustable](½ cup) and granulated sugar (¼ cup)[/adjustable] in a mixing bowl and use a wooden spoon or spatula to stir them together (*do not use a mixer!).
11 Next, stir in the egg [adjustable](1 large) and vanilla extract (2 teaspoons), followed by the baking soda (½ teaspoon) and salt (½ teaspoon)[/adjustable]. Mix in unsweetened dark cocoa powder [adjustable]½ cup) and all-purpose flour (1 cup)[/adjustable] until well-combined, then stir in white chocolate chips (1 cup).
4 Immediately spread dough into a greased cookie cake pan (you don't want the dough sitting out or it will set too much with the melted butter).If you plan to remove the cookie cake from the pan for serving, I also recommend lining the bottom with parchment paper.
6 Bake at 350°F for 12-14 minutes or until cookie cake looks “rippled” and the center no longer looks super wet (see photo in post above!). Cool completely before frosting.
7 Using a large star piping tip, pipe around the edges then immediately add the mini Oreos to decorate. The frosting will start hardening quick, so add the Oreos right away!
To Make Frosting:
10 Beat the unsalted butter ([adjustable]½ cup, i.e. 1 stick), cream cheese (4 ounces), vanilla extract (2 teaspoons[/adjustable]), and a pinch of salt on high speed for 1-2 minutes until combined. (It may look a little curdled, but this is okay!) Turn the mixer to low speed then slowly add the powdered sugar ([adjustable]2 ½ cups[/adjustable]). You may need to stop and scrape down the sides.
12 Beat on high for 1-2 minutes until creamy and the frosting is at a good consistency to pipe. If it looks too runny, add more powdered sugar. If it looks too stiff, add a tablespoon of milk until you get to the desired consistency. Fold in the crushed Oreos (¼ cup). 
13 This will make enough frosting to pipe a generous border along the edges and a message in the center.