Preheat your oven to 350°F (175°C) and prepare a 9x13 inch baking pan by greasing it and lining the bottom with parchment paper.
In a large saucepan, melt 1 cup of unsalted butter over low heat. Remove from heat and whisk in the granulated sugar until smooth. Allow to cool for a few minutes.
Add the eggs one at a time, whisking them in thoroughly along with the pure vanilla extract until combined.
In a separate bowl, sift together the all-purpose flour, unsweetened cocoa powder, salt, and baking powder. Gradually incorporate this dry mix into the wet ingredients, stirring gently until just combined.
Fold in the 10 crushed Oreo cookies until evenly distributed.
Pour the brownie batter into the prepared baking pan and spread it out evenly. Bake for 25-30 minutes, checking doneness with a toothpick.
In a medium mixing bowl, beat the softened butter until creamy. Gradually add the powdered sugar, mixing on low speed until incorporated. Add the pure vanilla extract and milk, beating until smooth. Fold in the crushed Oreo cookies.
Once the brownies are cool, spread the Oreo frosting evenly across the top.
Crush a few more Oreo cookies and sprinkle them over the frosted surface.
Cut the brownies into squares and serve at room temperature.
Notes
Serve on a colorful platter with whole Oreos for decoration.