Prep the pan and oven: Heat the oven to 350°F (175°C). Line an 8-inch or 9-inch square pan with parchment, leaving overhang for easy lifting. Lightly grease the parchment.

Melt the butter and chocolate: In a microwave-safe bowl, melt butter and half of the chocolate chips in 20–30 second bursts, stirring until smooth. Let it cool for 3–4 minutes so it doesn’t scramble the eggs.

Whisk in sugars and eggs: Add granulated sugar and brown sugar to the warm mixture and whisk until glossy. Add eggs, one at a time, whisking well after each. Stir in vanilla. The batter should look thick and shiny.

Mix the dry ingredients: In a separate bowl, whisk flour, cocoa powder, salt, and espresso powder if using. Fold the dry ingredients into the wet until just combined. Don’t overmix.

Add the mix-ins: Fold in the remaining chocolate and about two-thirds of the chopped Oreos. The batter will be very thick—this is good.

Spread and top: Scrape batter into the pan and smooth the top. Press the remaining Oreos lightly into the surface. Sprinkle with Halloween sprinkles for a festive finish.

Bake: Bake 28–35 minutes for an 8-inch pan or 22–28 minutes for a 9-inch pan. You’re looking for set edges and a center that’s just slightly soft. A toothpick should come out with a few moist crumbs, not wet batter.

Cool completely: Let the brownies cool in the pan on a rack for at least 1–2 hours. For the cleanest slices, chill for 30–60 minutes after they reach room temperature.

Finish and slice: If you like, add a pinch of flaky sea salt on top. Use the parchment to lift the slab out and cut into 16 squares with a sharp knife, wiping the blade between cuts.