Preheat your oven to 350°F (177°C). Place a rack in the center position. Line a 9x13-inch metal baking pan with parchment paper, leaving about 1 inch of overhang on the long sides. Lightly grease the parchment. Checkpoint: You are ready to move on when the pan is lined, greased, and the oven light shows it has reached temperature.

In a medium saucepan, combine the butter and chopped baking chocolate over low heat. Stir constantly for 5–7 minutes until fully melted and smooth. Remove from heat and let the mixture cool for 5–10 minutes — it should no longer feel hot when you hold your hand near it and should not be steaming. Checkpoint: You are ready to move on when the mixture is glossy, smooth, and warm but not hot to the touch.

In a large mixing bowl, add the eggs, granulated sugar, and brown sugar. Using a hand mixer on medium-high speed, beat for a full 4 minutes. The mixture will turn pale yellow, roughly double in volume, and fall off the beaters in thick, slow ribbons. Do not rush this step — it creates the shiny crackle top. Checkpoint: You are ready to move on when the mixture is visibly pale and leaves a thick ribbon trail that holds for 3–4 seconds before dissolving.

Pour the cooled chocolate-butter mixture into the egg-sugar bowl. Add the vanilla extract and espresso powder. Whisk by hand for 1–2 minutes until the batter is dark, uniform, and glossy with no streaks. Checkpoint: You are ready to move on when the batter is one smooth, shiny color with no separate layers visible.

Sift the cocoa powder, flour, and salt directly over the batter. Switch to a rubber spatula and fold gently — use slow, sweeping strokes from the bottom of the bowl upward. Fold only until no white flour streaks remain, about 30–60 seconds. Stop immediately — overmixing builds gluten and makes brownies rubbery. Checkpoint: You are ready to move on when the batter is thick, glossy, and completely uniform with zero dry patches.

Add the semisweet chocolate chips and fold them in with the spatula for about 30 seconds. Pour the batter into the prepared pan and spread it into an even layer using an offset spatula or the back of a spoon. The surface should be smooth and level.

Scatter Halloween sprinkles evenly over the top. Press candy googly eyes and Halloween candies (M&Ms, candy corn, or mini Reese's) gently into the surface. Work quickly so the batter does not start to set. Place the pan in the center rack of the oven.

Bake at 350°F for 28–32 minutes. At the 14-minute mark, rotate the pan 180° for even baking. Starting at 25 minutes, begin testing with a toothpick inserted into the center — you want it to come out with moist fudgy crumbs attached, NOT wet liquid batter and NOT completely clean. Edges should be set and pulling slightly away from the pan sides. Checkpoint: You are ready to pull from the oven when the toothpick shows moist crumbs and the center looks set but still slightly glossy on top.

Remove the pan from the oven and set it on a wire cooling rack. Do not cut yet. Cool completely in the pan for at least 1–2 hours. The center will firm up significantly as it cools. Checkpoint: You are ready to move on when the brownies feel firm throughout when pressed gently and are no longer warm in the center.

Optional decoration: Melt white chocolate and transfer to a piping bag or zip-lock bag with a tiny corner snipped. Pipe mummy-bandage lines or spider-web patterns over the cooled brownies. Add additional candy googly eyes. Let the white chocolate set for 10–15 minutes until firm and opaque.

Lift the brownies out of the pan using the parchment overhang. Place on a cutting board. Slice into 24 squares using a plastic knife or a sharp chef's knife dipped in hot water and wiped dry between cuts for clean edges.