Preheat oven to 350°F. Line an 8x8 inch baking pan with parchment paper, leaving overhang for easy lifting.
In a large bowl, whisk melted butter, granulated sugar, and brown sugar until glossy and combined.
Whisk in eggs one at a time, then add vanilla. Beat vigorously for 0.5 to 1.0 minute until the batter thickens and lightens slightly.
In a separate bowl, whisk flour, cocoa powder, salt, and espresso powder.
Fold dry ingredients into the wet mixture just until no dry streaks remain. Do not overmix.
Fold in chocolate chips and 2.0 cups chopped Oreos.
Spread batter evenly into the prepared pan. Scatter the remaining 0.5 cup Oreo pieces over the top and gently press them in.
Bake for 28 to 32 minutes, or until the edges are set and a toothpick inserted 2.0 inches from the edge comes out with moist crumbs (not wet batter).
Cool in the pan on a rack for at least 30 minutes. Sprinkle with flaky sea salt if using, then lift out and cut into 16 squares.
Notes
For clean slices, chill the brownies for 20 minutes before cutting. Store airtight at room temperature for 3 days or refrigerate up to 5 days. Warm briefly before serving for maximum fudginess.