Make the cookie dough: Beat butter and white sugar in a mixing bowl until smooth. Add eggs and mix until incorporated. Add flour, baking soda, and salt; mix to a sticky, uniform dough. Fold in chocolate chips gently. Press dough evenly into the prepared pan using damp hands or a small offset spatula.

Arrange whole Oreo cookies in a single layer on top of the cookie dough to cover completely.

Melt chocolate and butter together in a microwave using 30-second pulses until smooth. Whisk in sugar, then add eggs and mix until uniform. Fold in flour, cocoa, and salt until batter is smooth.

Pour brownie batter gently over the Oreo layer, spreading evenly. Bake for 40–50 minutes, or until a skewer inserted comes out with moist crumbs. Avoid overbaking to keep brownies fudgy.

Freeze brownies for about 3 hours. Once frozen, remove from the pan and cut into squares. Store in an airtight container in the refrigerator for up to 1 week.