Preheat oven to 325℉. Prepare an 8x8 making dish by lining it with parchment paper. In a medium saucepan, melt the butter over medium heat. Once melted, remove from heat. While butter is hot, add the oil, granulated sugar, and brown sugar, whisking vigorously to combine. Pour mixture into a large mixing bowl and let cool for 5-10 minutes.

Once the mixture has cooled down a bit, add the vanilla, eggs, and egg yolk. Whisk again vigorously to combine, until mixture is glossy and smooth. Add the cocoa powder and whisk to combine. Add the flour, baking powder, and salt, and fold together with a rubber spatula until combined. Pour batter into the prepared baking pan. Bake for 32-35 minutes or until the center is set and a toothpick inserted in the center comes out clean. Let cool for 30 minutes in the pan, then lift the brownies out of the pan onto a cooling rack, and let cool completely.

Once brownies have cooled, make the buttercream. Add the softened butter to the bowl of a stand mixer fitted with the paddle attachment. You can also use a large mixing bowl and electric hand mixer. Cream the butter for about 2 minutes on medium-high speed, until light and fluffy. Lower the speed and add the powdered sugar gradually. Once incorporated, increase speed to medium-high and beat for another 2 minutes, scraping down the sides as needed. Add the vanilla and heavy cream (starting with 1 tablespoon and adding more if needed), and beat again on high speed for 1 minute. Add the crushed Oreos, and beat on high speed until incorporated.

Frost the brownies with a thick later of buttercream, using an offset spatula to smooth out the top. You won't use all of the buttercream, there will likely be 1/2-1 cup leftover. Cut the brownies into 9 squares, then garnish the top of each with a few pieces of crushed Oreo. Enjoy!