Preheat oven to 350F. Line a 9x9-inch pan with parchment paper, leaving an overhang on two sides for easy removal. Lightly grease any exposed pan surface.

In a large microwave-safe bowl, combine butter and 1 cup chocolate chips. Microwave in 30-second bursts, stirring between each, until smooth and glossy. Let cool 5 minutes so you don't scramble the eggs.

Whisk sugar into the melted chocolate until well combined. Add eggs one at a time, whisking vigorously after each, then stir in vanilla. Sift flour, cocoa powder, and salt over the mixture. Fold gently with a spatula until just combined and no dry streaks remain. Fold in remaining 1/2 cup chocolate chips.

Spread half the batter in the prepared pan. Arrange Oreos in a 4x4 grid, pressing them slightly into the batter. Dollop remaining batter over the top and spread carefully to cover the cookies completely.

Bake 32-36 minutes until the top is set and a toothpick inserted near the edge comes out with moist crumbs. The center should still look slightly underdone - it will firm up as it cools. Let cool completely in the pan, at least 2 hours, before lifting out and cutting into 16 squares.