Preparation Steps

• Preheat the oven to 350°F. Line a 9x13-inch pan with parchment paper leaving a 1–2 inch overhang to lift the finished bars out, and lightly grease the exposed sides.
• Make the cookie dough: In a bowl, beat the softened butter with the brown and granulated sugars until smooth and slightly fluffy, about 2–3 minutes. Add the egg and vanilla and beat until incorporated.
• Whisk together flour, baking soda, and salt in a separate bowl. Fold the dry mix into the butter mixture until just combined, then gently stir in the chocolate chips. If the dough seems very soft, chill it 10–15 minutes to make it easier to press into the pan.
• Press the cookie dough into the prepared pan in an even layer, covering the bottom but leaving the edges slightly thinner so the Oreo layer will sit flush.
• Arrange the Oreos in a single snug layer over the cookie dough. Break a couple to fill gaps at the edges so the Oreo layer reaches corner-to-corner.
• Prepare the brownie layer: Place the chopped chocolate and butter in a medium heatproof bowl and melt together over a gentle simmer, stirring until smooth. Remove from the heat, stir in espresso powder if using, and let cool 5 minutes.
• In a clean bowl, whisk the eggs and granulated sugar vigorously until the mixture is pale and has thickened slightly (about 3–5 minutes by hand or 2–3 minutes with a mixer). Stir in the vanilla, then gradually add the slightly cooled chocolate-butter mixture and mix until even.
• Sift the cocoa, flour, and salt into the chocolate mixture and fold gently just until no streaks remain. Fold in the 1/2 cup chocolate chips.
• Carefully pour the brownie batter over the Oreo layer, spreading with a spatula to cover without digging down to the cookie layer. Sprinkle a few extra chocolate chips over the top if desired and finish with a light sprinkle of flaky sea salt.
• Bake in the preheated oven for 32–40 minutes. Start checking at 32 minutes: the top should be set with a few moist crumbs on a toothpick but not wet batter. If you prefer fudgier bars, err on the shorter bake time.
• Cool completely in the pan on a wire rack — at least 1 hour. For cleaner slices, chill 30–60 minutes in the refrigerator before lifting the slab out on the parchment and cutting into 24 bars.