1. Make the Brownie Layer:

• Preheat and Prep: Preheat oven to 325°F (163°C). Grease an 8×8 inch baking pan and line it with parchment paper, leaving an overhang on two sides for easy removal. Set aside.  
• Combine Fats and Sugars: In a medium saucepan, melt the 1/2 cup of salted butter over medium heat. Once melted, remove from heat. Immediately whisk in the vegetable oil, granulated sugar, and brown sugar vigorously until combined.
• Cool Mixture: Pour the butter/sugar mixture into a large mixing bowl and let it cool for 5-10 minutes (it should be warm, not hot).
• Add Eggs and Vanilla: Add the vanilla extract, eggs, and egg yolk to the slightly cooled butter mixture. Whisk again vigorously until the mixture is glossy and smooth.
• Add Dry Ingredients: Add the cocoa powder, flour, baking powder, and salt to the bowl. Fold together gently with a rubber spatula until just combined. Do not overmix.
• Bake: Pour the batter into the prepared baking pan and spread evenly. Bake for 32-35 minutes, or until the center is set and a toothpick inserted into the center comes out with moist crumbs attached (you want them slightly fudgy).
• Cool Completely: Let the brownies cool in the pan on a wire rack for at least 30 minutes. Then, use the parchment paper sling to lift the brownies out of the pan onto the cooling rack. Let cool completely before frosting.

2. Make the Cookies and Cream Buttercream:

• Cream Butter: Add the softened salted butter to the bowl of a stand mixer fitted with the paddle attachment (or use a large mixing bowl and an electric hand mixer). Cream the butter on medium-high speed for about 2 minutes, until light and fluffy.
• Add Sugar: Lower the mixer speed and gradually add the sifted powdered sugar, mixing until incorporated.
• Beat Frosting: Increase the speed to medium-high and beat for another 2 minutes, scraping down the sides of the bowl as needed, until very light and fluffy.
• Add Liquids & Vanilla: Add the vanilla extract and 2 tablespoons of heavy cream. Beat again on high speed for 1 minute until smooth. If the frosting seems too thick, add a tiny bit more cream (1 teaspoon at a time) until desired consistency is reached.
• Fold in Oreos: Add the 1 cup of crushed Oreos and beat briefly on low speed (or fold in by hand) until just incorporated.

3. Frost and Finish:

• Frost Brownies: Once the brownies are completely cool, spread a thick layer of the cookies and cream buttercream over the top using an offset spatula to smooth it out. (Note: You may have 1/2-1 cup of leftover frosting).
• Cut and Garnish: Cut the frosted brownies into 9 squares. Garnish the top of each brownie with a few pieces of extra crushed Oreo. Enjoy!