Preheat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper and lightly grease.
In a large bowl, mix melted butter and sugar until smooth. Add eggs and vanilla and beat until combined.
Stir in cocoa powder, flour, and salt. Fold in chopped Oreos.
Spread half the brownie batter into the prepared pan.
In a separate bowl, beat cream cheese until smooth. Add sugar, egg yolk, and vanilla, and mix until creamy.
Spoon cream cheese mixture over the brownie layer and spread gently.
Top with remaining brownie batter in spoonfuls, then swirl gently with a knife to marble.
Press halved/chopped Oreos into the top.
Bake for 35–40 minutes, or until the center is just set.
Cool completely before slicing into bars.