Preheat your oven to 350°F (175°C).

Line an 8x8-inch (or 9x9-inch) metal baking pan with parchment paper, leaving overhang on two sides to easily lift the brownies out later. Lightly grease the paper.

In a medium bowl, beat the softened butter, brown sugar, and granulated sugar together with a hand mixer (or whisk) until smooth and creamy.

Whisk in the egg and vanilla extract.

Stir in the flour, baking soda, and salt just until combined. Fold in the chocolate chips.

Press the cookie dough evenly into the bottom of the parchment-lined pan.

Arrange the Oreo cookies in a single, tight layer directly on top of the raw cookie dough layer (a 4x4 grid usually fits perfectly).

In a heatproof bowl, combine melted butter, granulated sugar, cocoa powder, and salt. Whisk until smooth.

Add the eggs one at a time, whisking vigorously after each addition until the batter becomes glossy and thick. Stir in the vanilla.

Gently fold in the flour using a spatula until just incorporated (do not overmix). Fold in the dark chocolate chunks.

Pour the brownie batter over the Oreo layer, using an offset spatula to spread it evenly so all cookies are completely covered.

Bake at 350°F (175°C) for 40 to 45 minutes, or until a toothpick inserted into the brownie layer comes out with a few moist crumbs attached (not wet batter).

Remove from the oven and allow to cool completely in the pan on a wire rack for at least 2 hours.