Preheat the oven to 350°F (convection).

In a medium bowl, mix together the room-temperature butter and granulated sugar until combined.

Add the eggs and egg yolks. Whisk for 2–3 minutes, until the mixture turns light and off-white.

(This step is key for that classic crackly brownie top!)

Stir in the vegetable oil and vanilla extract until fully incorporated.

Stir in the cocoa powder, all-purpose flour, cornstarch, and baking soda until just combined.

Line an 8×8-inch square pan with parchment paper, then pour the batter evenly into the pan.

Bake for 28–32 minutes, until the edges are set and the center is just slightly soft.

Let the brownies cool in the pan for about 10 minutes, then transfer to the refrigerator to chill for about 15 minutes.

(This step helps prevent crumbling when slicing.)

While the brownies chill, beat the room-temperature butter in a separate bowl until light and off-white. Add the powdered sugar and mix until fully incorporated and slightly thick.

Mix in the heavy cream and vanilla extract, then gently fold in the crushed Oreos.

Once the brownies are fully chilled, spread the Oreo frosting evenly over the top.

Cut into 16 squares (4×4) and enjoy! Store leftovers in an airtight container in the refrigerator for up to 10 days.