1. Preheat oven to 350°F (175°C) and line a 9×9-inch pan with parchment paper.

2. Melt butter and dark chocolate together until smooth, then let cool slightly.

3. Whisk granulated sugar, brown sugar, and eggs until light and fluffy.

4. Stir in the cooled chocolate mixture and vanilla extract until combined.

5. Sift flour, cocoa powder, and salt together, then fold into the wet mixture until just combined.

6. Pour batter into the prepared pan and smooth the top.

7. Bake for 25–30 minutes or until a toothpick yields moist crumbs, then cool completely on a rack.

8. Combine powdered sugar and milk to form smooth icing, adding white chocolate if desired.

9. Pipe concentric circles of icing on the brownie surface.

10. Drag a toothpick from center to edge at intervals to create a spiderweb pattern.

11. Top with candy spiders, edible glitter, or red gel “blood” for decoration.