Position a rack in the center of the oven and heat to 350°F. Line a 9x9-inch metal baking pan with parchment paper, leaving an overhang on two sides for easy lifting.

In a large bowl, whisk the melted butter with the granulated sugar and brown sugar until smooth. Whisk in the eggs and vanilla. Add the cocoa powder, flour, and salt, and stir with a spatula until just combined — a few streaks are fine; don’t overmix, which keeps the centers fudgy. Fold in the chocolate chips. Spread evenly in the prepared pan.

Bake until the edges are set and a toothpick inserted in the center comes out with moist crumbs (not wet batter), 28–32 minutes.

Set the pan on a wire rack and cool completely, about 1 hour. Lift the slab out using the parchment overhang and cut into 12 squares. The brownies must be fully cool or the icing will melt on contact.

Whisk the powdered sugar with 2 tablespoons of the milk and the vanilla until smooth and thick but pipeable — it should ribbon off the whisk. Add the remaining tablespoon of milk only if needed. Transfer to a piping bag (or a zip-top bag with a small corner snipped).

For 6 brownies: pipe 3–4 concentric circles of icing on each square. Drag a toothpick from the center outward through the circles (about 8 lines) to pull the web shape. Top with a candy spider if you like.

For the remaining 6 brownies: pipe horizontal zigzag bands of icing across each square, leaving two small gaps. While the icing is still wet, press 2 candy eyes into each gap so they peek through the “bandages.” Let the icing set about 30 minutes before stacking or packing.