Preheat the oven to 350°F (175°C). Line a 9×13-inch baking pan with foil or parchment paper and lightly spray the lining with nonstick cooking spray.

Place the chopped unsweetened baking chocolate and the sliced unsalted butter in a large microwave-safe bowl.

Microwave on high in 30-second increments, stirring between each increment, until the chocolate and butter are fully melted and the mixture is smooth (about 2 minutes total, depending on your microwave).

Stir the granulated sugar and the unsweetened cocoa powder into the melted chocolate and butter until combined.

Add the eggs (one at a time if you prefer) and the vanilla extract, stirring after each addition until incorporated. Stir in the ½ teaspoon salt. The batter may look slightly grainy but should be smooth overall.

Gently fold in the all-purpose flour until just combined and no streaks of flour remain. Do not overmix.

Pour half of the batter into the prepared pan and spread it into an even layer using a spatula.

Arrange all 24 Oreo cookies in a single even layer over the batter. Space them so they cover the batter as uniformly as possible.

Spoon the remaining batter over the Oreos and gently spread to cover them. It’s okay if some cookies remain partially visible; try to create an even top layer without forcing the batter too thin.

Bake in the preheated oven for 25–35 minutes. The brownies are done when the top is no longer glossy and a toothpick inserted about 2 inches from the side comes out with just a few moist crumbs.

Remove the pan from the oven and let the brownies cool completely in the pan. Use the foil or parchment to lift the brownies from the pan before slicing.