Heat the oven to 350°F (175°C). Line a 9 x 13-inch (23 x 33 cm) baking pan with parchment paper, leaving enough overhang to lift out the cooled brownies.

Whisk the melted butter and granulated sugar together in a large bowl. Add the eggs and vanilla extract, then whisk until the mixture looks smooth and evenly combined.

In a separate bowl, combine the flour, unsweetened cocoa powder, baking powder, and fine salt. Add the dry ingredients to the butter mixture and fold just until no dry streaks remain.

Stir in the semisweet chocolate chips if using, taking care not to overmix the batter. Spread the batter evenly into the prepared pan.

Bake for 25 to 32 minutes, until the center is set but still moist. A toothpick inserted in the center should come out with a few damp crumbs rather than wet batter.

Cool the brownies completely in the pan. For cleaner cuts and easier decorating, refrigerate them for 30 minutes once cooled.

Melt the white chocolate or candy melts and pipe ghost shapes, spiderwebs, zigzags, or a thin frosting-style layer over the brownies. Melt the dark chocolate and pipe web lines or ghost faces.

Add candy eyes and orange or black sprinkles while the coating is still soft. Let the decoration set, then lift the brownie slab from the pan and cut it into squares or Halloween shapes.