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

Melt the butter. In a heatproof bowl, melt the butter until fully liquid. Stir in the oil. Let cool slightly so it’s warm, not hot.

Whisk sugars and eggs. In a separate bowl, whisk granulated sugar, brown sugar, eggs, and egg yolk for 1–2 minutes until slightly thickened and glossy. This helps create that shiny top.

Add vanilla and cocoa. Whisk in vanilla, then sift in cocoa powder and espresso powder. Mix until smooth and dark.

Combine with butter mixture. Pour the warm butter-oil mixture into the cocoa batter and whisk until fully incorporated.

Fold in dry ingredients. Sift in flour and salt. Switch to a spatula and fold gently until no dry streaks remain. Do not overmix. Fold in chocolate chips.

Spread and bake. Transfer batter to the prepared pan and smooth the top. Bake 22–28 minutes, until the edges are set and a toothpick inserted 2 inches from the edge comes out with moist crumbs, not wet batter.

Cool completely. Set the pan on a rack and cool fully. This sets the fudgy texture and makes clean slices.

Decorate. Lift brownies from the pan. For a quick “mummy” effect, melt white chocolate with a splash of milk or cream and drizzle in zigzags. Add candy eyes so they stick while the drizzle is soft. Press in sprinkles, cookie crumbs, M&M’s, or peanut butter cup “gravestones.” For “witch brooms,” poke pretzel sticks into mini peanut butter cups and place them on top of cut squares.

Slice smart. Use a warm knife (wipe between cuts) for neat edges. Cut into 12–16 squares.