1. Preheat your oven to 350°F (175°C). Line an 8×8 inch baking pan with parchment paper, leaving an overhang on two sides to easily lift the brownies out later. Set aside.

2. In a microwave-safe bowl, combine the butter, vegetable oil, granulated sugar, and brown sugar. Microwave in 30-second intervals, stirring vigorously after each, until fully melted and combined. Continue to microwave for an additional 1 minute and stir vigorously again to ensure the sugar is fully dissolved. This step is crucial for that shiny, crinkly top!

3. Whisk in the cold eggs one at a time, ensuring each egg is fully incorporated before adding the next. The batter should start to thicken and pull away from the bowl slightly, indicating aeration. Stir in the vanilla extract.

4. Add the all-purpose flour, cocoa powder, and salt to the wet ingredients. Gently fold just until combined. Be careful not to overmix, as this can lead to tough brownies.

5. Pour approximately half of the brownie batter into the prepared baking pan. Spread it evenly across the bottom using an offset spatula or the back of a spoon.

6. Arrange the 15 whole mint Oreos evenly over the first layer of batter in the pan. Gently press them down slightly. Pour the remaining brownie batter over the Oreos and carefully spread it to cover them completely.

7. Crumble the extra 5 mint Oreos and sprinkle them over the top of the brownie batter. Bake in the preheated oven for 30 minutes. The edges should be set, and a toothpick inserted into the center should come out with moist crumbs, not wet batter.

8. This step is crucial for perfect slices! Allow the brownies to cool down completely on a wire rack before attempting to slice them. Cooling completely allows the brownies to set properly, resulting in clean, perfect squares with that coveted crinkle top.