Preheat the oven to 350°F / 175°C and line an 8 x 8-inch square pan with parchment paper. Set aside.
Mix 1 ½ cups of crushed Oreo cookie crumbs with ¼ cup melted butter. Press into the prepared pan and smooth using the back of a large spoon.
Bake for 8 minutes, then remove from the oven. Leave the oven while you make the brownie and cheesecake batter.
Place 2 large eggs, ½ cup of granulated sugar, and ½ cup of brown sugar in a large bowl and mix on high for a minute with a hand mixer or stand mixer. The mixture should get lighter in color and look creamy.
Add ½ cup softened butter and 2 tsp vanilla extract to the egg mixture. Mix on a medium speed until combined and smooth.
Sift ½ cup all-purpose flour, ⅓ cup cocoa powder, and ¼ tsp baking powder into the egg mixture and mix on a low speed. The batter should be somewhat thick at this point. Set aside.
In a medium-sized bowl beat 8 oz. or 1 cup of room temperature cream cheese on a medium-high speed with a hand mixer or stand mixer fit with a whisk attachment for a minute until it's creamy and smooth.
Gradually mix in ⅔ cup of sugar and 1 tsp of vanilla. Mix on a medium-low speed until incorporated.
Then mix in 1 egg at medium-low speed until combined. Scrape the sides and bottom of the bowl as needed with a rubber spatula. At this stage, the cheesecake batter should look silky smooth.
Fold in ½ cup of roughly chopped Oreos. Mix until they're evenly distributed throughout the batter.
Alternate spoonfuls of brownie batter and cheesecake filling on top of the cooled Oreo crust. Use a butter knife or small offset spatula to swirl the batter together.
Bake at 350 F / 175 C for 40-45 minutes or until the edges of the brownies look set. The toothpick test won't work with these because of the cheesecake swirl, so I like to do a jiggle test. The edges should be set but the center can still be a little wiggly once they're done. In my oven, the sweet spot is 44 minutes, but every oven bakes a bit differently! If you notice the cheesecake swirl is browning towards the end of the baking process, place a piece of foil over the pan.
Remove the pan from the oven and cool fully before cutting. You can accelerate the cooling process by popping the pan in the freezer for about an hour. I like these best when they're chilled and straight out of the fridge!
Seasonal variation:
For the brownies, I used ½ cup of brown butter in place of the regular butter, ½ cup of pumpkin puree (125g) in place of the 2 eggs, and dark brown sugar instead of light brown sugar.
For the cheesecake, I added ¼ cup of pumpkin puree in place of the egg. I then divided the batter in half and added a bit of orange gel food coloring to one portion
